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Last updated: 4 February 2026

The latest on the stench

As you know, over the past few weeks, we’ve experienced significant challenges with the Christchurch Wastewater Treatment Plant's oxidation ponds – unexpectedly, conditions are the worst they've been since the fire.

Usually at this time of year, the ponds are healthy, but several compounding factors have contributed to the stench.

How we got here

Each year, the wastewater load which arrives over the holiday period varies significantly as industry shuts down for the summer holiday and our population varies. The temporary activated sludge plant, which is a biological process, doesn't manage this variation very well.

This can result in the temporary plant struggling and passing additional load to the ponds, however, usually at this time of year the ponds are very healthy and are able to pick up the slack.

On top of this, we expected the high levels of rain in January to help flush wastewater through the system and improve pond health; however, this didn't happen, as it appears to have set back the algae growth in the ponds.  

While we know the weather and high load are contributing factors, our team is reviewing all data to understand whether other factors also played a role in the current situation.

We’ve always known we're operating the temporary plant with a narrow margin for error, as it has no extra capacity. This makes the system more vulnerable to sudden changes. While the ponds are now showing signs of improvement, much of the recovery depends on natural processes such as algae growth and oxygenation.

Despite these challenges, we're using every tool available and are seeing progress. Increasing aeration in the ponds remains a key focus.

What we’re doing

  • Balancing flow between ponds to better distribute load and support recovery.
  • Regular jetboat oxygenation to increase dissolved oxygen.
  • Ongoing monitoring of key pond health indicators, including dissolved oxygen levels, drone imagery, and lab samples (with a focus on algae levels, a major oxygen source).
  • Updating our odour mitigation plan to reflect the current event.
  • Doubling our odour assessment frequency, with an external contractor now reporting twice weekly (these graphs will be updated to our website).
  • Dosing the ponds with hydrogen peroxide to support pond oxygenation.
  • Monitoring the weather ahead of the incoming cold, wet system to help determine the best way to manage flows through the ponds.

Other options we’re considering

  • Methods to boost oxygen in the temporary activated sludge plant.
  • Specialist algae sampling to help us further understand the situation.  

Recap - Residents meeting 

Thank you to everyone who came along to the meeting. 

We want to again acknowledge the hardship the community has faced and the impact the ongoing stench has had on daily life - not only in recent weeks, but throughout the years since the fire. We are committed to improving our processes so we can reduce the risk of similar events happening again, and provide earlier warnings when conditions change in the future.

The Mayor and Council staff gave an update on how this situation arose. We also heard clearly the call for better engagement. Your feedback is guiding our next steps.

Come and see us on 5 February 

We'll have staff on the corners of Carters and Shortland streets tomorrow. Look out for the Council-branded trailer and feel free to come and speak one-on-one with members of our Community Partnerships and Three Waters teams. 

Door knocking next week 

Next week, we'll be in the community visiting neighbours of the plant to hear from them directly about their priorities. 

The latest on pond health and odour 

We’ve been out on site again today and are pleased to report further improvements across the pond system with odour levels dropping.

Following yesterday’s rain, flows through the ponds have increased, and we’re seeing positive changes in the pond colour, which is shifting back toward a healthier state. There is also a noticeable amount of algae returning around the edges of Pond 2, which is an encouraging sign of recovery. All aerators are operating as they should.

Looking ahead, the expected sunny conditions should support the continued return of algae and help the ponds regain their natural balance.

While staff are cautiously optimistic about recent improvements in pond health, the ponds remain in a delicate and changeable state. Current odour monitoring is showing low levels of odour, however, under certain weather and operating conditions there remains a risk of odour returning.

We're meeting with external experts tomorrow to identify ways to strengthen the future resilience of the pond system and reduce the risk of similar events happening again.

Long-term solution

Once commissioned, the new activated sludge reactor will address the long‑standing odour issues that followed the fire. We are actively looking for opportunities to accelerate this programme wherever possible, though the timeline is already extremely tight. We’ll continue to make the most of any chance to bring that work forward.

Our contractors are already busy getting construction started, and the programme is expected to take about three years, with commissioning targeted for 2028. 

We know this is a long time to wait, and while we construct the new plant, we're doing everything we can to keep the fire-damaged plant and oxidation ponds operating as well as they can. 

Health, wellbeing and support services

We’re sorry for the impact this continues to have on you and your whānau. Please be assured we are doing everything we can to support the system’s recovery and reduce odour where possible.

Dr Annabel Begg, Medical Officer of Health for the National Public Health Service has provided the advice below:

"Exposure to hydrogen sulphide odour arising from the Christchurch wastewater treatment plant can cause nausea, headaches, eye and throat irritation, skin irritation, sleep disturbance, and worsening asthma symptoms at relatively low concentrations.

If people exposed to the odour don't experience physical health effects, continued exposure to unpleasant or nasty, noxious odours can still have an adverse effect on people's mental wellbeing.

While exposure to hydrogen sulphide is unpleasant and may result in health effects, it doesn’t accumulate in the body. Any health effects would be expected to resolve when the odours are brought back to normal levels, and long-term health effects are highly unlikely.

People experiencing health issues as a result of the odour coming from the treatment plant should seek health advice from their GP, doctor, Hauora or healthcare provider, or call Healthline free on 0800 611 116, anytime 24/7 for advice as soon as possible. In an emergency call 111."

There are also free services across Ōtautahi Christchurch available to support and enhance overall health and wellbeing. These include:

Health Improvement Practitioners (HIPs)

HIPs are qualified mental health professionals based in General Practices (GPs). They offer free, 15 to 30-minute sessions focused on practical strategies to support mental and physical wellbeing. There is no limit to the number of sessions you can access. You can book directly through your GP.

Pae Ora ki Waitaha Support

Pae Ora ki Waitaha is a free healthy lifestyles service developed with Māori and Pasifika providers.

More information is available on their website(external link).

Healthline

Healthline is a free over-the-phone health service available 24 hours a day, seven days a week. If you or someone in your family is unwell, you can call Healthline and speak to experienced nurses and paramedics for health advice and information about what to do next.

Call 0800 611 116.

Timeline of key events and decisions

November 2021

  • On 1 November 2021, emergency services respond to a fire at the Wastewater Treatment Plant in Bromley. Both of the trickling roof filters collapse. The damage couldn't be assessed properly until the fire was completely out.
  • On 4 November, the Council installed a sprinkler system to remove hot spots and help reduce the smell. We also begin working on an adaptive management plan to figure out the best possible outcome for the discharge of wastewater from the plant.
  • On 26 November, the fire is officially completely extinguished by the fire service. The Council identifies the need to remove the two trickling filters. Both filters store 13,000 cubic metres of material.
  • The Council started using misters and polyaluminium chloride - an odourless powder that dissolves in water - to help suppress the smell. Polydosing also settles more of the suspended solids in the wastewater, improving the water quality as it progresses through the plants.
  • Hydrogen peroxide - a compound that naturally breaks down into water and oxygen – starts to be added to the wastewater before it’s discharged into the oxidation ponds.

December 2021

  • On 6 December, the Council completed a successful trial to remove a section of the fire-damaged media from the plant. This means the Council could begin assessing the stability of the structures.
  • On 17 December, Council staff planned to convert two of the plant’s four clarifier tanks (or secondary contact) into aeration tanks. Installing four aerators in each of the two tanks will help decrease the bad smells and improve the quality of the wastewater being discharged. The Council anticipated that it would be years before the trickling filters could be replaced or rebuilt.

February 2022

  • On 14 February, aeration components begin to arrive to implement the interim plan to minimise odours from the fire-damaged plant.

March 2022

  • On 16 March, eight aerators are installed in two of the plant’s four clarifiers.
  • Soon afterwards, increased odours from the trickling filters are detected after wet weather. It's determined that the acute stench is caused by the material inside the trickling filters rotting and drying out.

April 2022

  • On 6 April, the aerators installed in March are turned on.
  • On 12 April, the Council decided to fast-track the appointment of a contractor to remove the burnt filter material.

May 2022

  • Filter media removal work starts.
  • On 6 May, independent testing to better understand the odours from around the fire-damaged plant detects small amounts of hydrogen sulphide and methyl mercaptan.
  • On 26 May, the Council confirms a community support package for 3,380 of the most affected residents.

June 2022

  • Southern Demolition and Salvage Limited conducted a test run, removing and transporting the burnt trickling filter material on Friday 3 and Saturday 4 June.
  • Work to remove the trickling filter began on 6 June.

July 2022

  • The new activated sludge treatment process becomes fully operational at the plant.
  • This new system pumps wastewater from the converted aeration basins to the clarifiers, which greatly improves the quality of the wastewater.

August 2022

  • Trickling filters are emptied one month ahead of schedule.

September 2022

  • Odours from the oxidation ponds are expected to reduce from September, although it is expected that some odours will remain for most of the month. These will progressively reduce as the pond's health improves.

April 2023

  • A project to increase the biological health of Pond 1 is completed after 16 aerators are installed and turned on, churning more than 300kg of oxygen into the pond every hour.
  • In the long term, these aerators will ensure the entire pond continues to stay healthy and function well.

May 2023

  • The disc aerators on Pond 1 failed and were in the process of getting repaired.

Late 2023

  • Fencing around oxidation ponds was installed to improve safety and reduce rubbish entering ponds. Subsequent minor changes were made to deter birds and improve effectiveness.
  • Insurance negotiations were continuing.
  • In November, a report on the insurance claim for the November 2021 fire was provided to the Council. 
  • In December, aerators on Oxidation Pond 1 restarted to minimise odours.

March 2024

  • A report on the insurance process and odour mitigation efforts was provided to the Council.
  • Odour mitigation plan released. 
  • Odour mitigation work is underway. Various actions taken to improve plant performance and reduce odour. 
  • A new interactive map was created showing odour mitigation work. 

April 2024

  • Ponds switched to winter operation mode. Adjustments made to our operations to reduce odours when the biology changes as weather cools. 

May 2024

  • The Mayor and councillors approved a grant of $130,000 to resource a project with the goal of strengthening and sustaining effective and respectful relationships with Eastern Communities.
  • Completed work to cover inlet chambers to contain odour. 
  • Completed replacement of the bio-bed material in the odour control system. Old material reused as soil conditioner for native planting areas.

October 2024

  • Christchurch City Council and its insurers have agreed on a settlement in relation to the fire-damaged trickling filters at the Christchurch Wastewater Treatment Plant.
  • The Council also approved the replacement of the trickling filters with an activated sludge reactor.
  • This significant milestone allows us to move forward with building a resilient and more environmentally friendly wastewater treatment plant for our growing city.
  • Replacing the trickling filters with an activated sludge reactor will give the Council the opportunity to significantly reduce greenhouse gas emissions. Additionally, activated sludge reactors can deliver better environmental outcomes and have a smaller footprint that can be cost-effectively upsized to accommodate the needs of our growing population.
  • Design work is underway and it’s expected to take up to three years to complete the design and construction of the new activated sludge reactors.

November 2024

  • Demolition of the Christchurch Wastewater Treatment Plant’s fire-damaged trickling filters started.
  • Southern Demolition is doing the work, which is expected to take four to five months to complete. 
  • The demolition is the first stage of physical work to replace the filters with an activated sludge reactor, which will fix long-standing odour issues. 

April 2025

  • Demolition of the trickling filters was completed making way for construction to begin on the the new activated sludge reactor later this year. 

February 2026

  • The bulk earthwork started. This will be completed in four stages and is expected to finish around early April 2026 .  

April 2026

  • The installation of approximately 1400 stone columns will begin. This will take approximately four months to complete.
  • Installation of the reactor discharge pipe and the Returned Activated Sludge (RAS) line to commence. This is expected to be completed in late October.

June 2026

  • Work to install an engineered gravel platform will begin.  

August 2026

  • The engineered gravel platform work is expected to be completed.
  • Construction of the concrete structure for the new reactor will begin. This is a major part of the project and will take around one year to complete.

October 2026

  • Installation of the reactor discharge pipe and the Returned Activated Sludge (RAS) line expected to be completed.
  • The process to install the supply pipes from the existing pump station to the new plant will begin. Due to the pipes’ depth, sheet piling will be required for all underground pipes. This will be completed around March 2027.

2027

  • Construction of the blower hall will commence. This will house the blowers and electrical room. It will be constructed with precast concrete panels. This will take around four months to complete.
  • The installation of the supply pipes from the existing pump station to the new plant will finish up around March.
  • The blower hall is expected to be completed around April.
  • Installation of all the equipment and pipes required to operate the new plant will commence. This will start with the blower hall fit-out and then move on to the reactor.
  • Pumps and pipework in the existing pump station will be replaced to accommodate the new plant.
  • Construction of the concrete structure for the new reactor is expected to be completed around August 2027.

2028

  • Commissioning the new plant and replacing the remaining two pumps in the pump station will begin. This will enable us to shut down the existing temporary activated sludge plant and prepare to return the two clarifiers to their original purpose.
  • Removing the temporary plant and making any repairs to the clarifiers and other equipment to return them to normal duty.
  • The entire project is expected to be completed in late 2028.

November 2021

  • On 1 November 2021, emergency services respond to a fire at the Wastewater Treatment Plant in Bromley. Both of the trickling roof filters collapse. The damage couldn't be assessed properly until the fire was completely out.
  • On 4 November, the Council installed a sprinkler system to remove hot spots and help reduce the smell. We also begin working on an adaptive management plan to figure out the best possible outcome for the discharge of wastewater from the plant.
  • On 26 November, the fire is officially completely extinguished by the fire service. The Council identifies the need to remove the two trickling filters. Both filters store 13,000 cubic metres of material.
  • The Council started using misters and polyaluminium chloride - an odourless powder that dissolves in water - to help suppress the smell. Polydosing also settles more of the suspended solids in the wastewater, improving the water quality as it progresses through the plants.
  • Hydrogen peroxide - a compound that naturally breaks down into water and oxygen – starts to be added to the wastewater before it’s discharged into the oxidation ponds.

December 2021

  • On 6 December, the Council completed a successful trial to remove a section of the fire-damaged media from the plant. This means the Council could begin assessing the stability of the structures.
  • On 17 December, Council staff planned to convert two of the plant’s four clarifier tanks (or secondary contact) into aeration tanks. Installing four aerators in each of the two tanks will help decrease the bad smells and improve the quality of the wastewater being discharged. The Council anticipated that it would be years before the trickling filters could be replaced or rebuilt.

February 2022

  • On 14 February, aeration components begin to arrive to implement the interim plan to minimise odours from the fire-damaged plant.

March 2022

  • On 16 March, eight aerators are installed in two of the plant’s four clarifiers.
  • Soon afterwards, increased odours from the trickling filters are detected after wet weather. It's determined that the acute stench is caused by the material inside the trickling filters rotting and drying out.

April 2022

  • On 6 April, the aerators installed in March are turned on.
  • On 12 April, the Council decided to fast-track the appointment of a contractor to remove the burnt filter material.

May 2022

  • Filter media removal work starts.
  • On 6 May, independent testing to better understand the odours from around the fire-damaged plant detects small amounts of hydrogen sulphide and methyl mercaptan.
  • On 26 May, the Council confirms a community support package for 3,380 of the most affected residents.

June 2022

  • Southern Demolition and Salvage Limited conducted a test run, removing and transporting the burnt trickling filter material on Friday 3 and Saturday 4 June.
  • Work to remove the trickling filter began on 6 June.

July 2022

  • The new activated sludge treatment process becomes fully operational at the plant.
  • This new system pumps wastewater from the converted aeration basins to the clarifiers, which greatly improves the quality of the wastewater.

August 2022

  • Trickling filters are emptied one month ahead of schedule.

September 2022

  • Odours from the oxidation ponds are expected to reduce from September, although it is expected that some odours will remain for most of the month. These will progressively reduce as the pond's health improves.

April 2023

  • A project to increase the biological health of Pond 1 is completed after 16 aerators are installed and turned on, churning more than 300kg of oxygen into the pond every hour.
  • In the long term, these aerators will ensure the entire pond continues to stay healthy and function well.

May 2023

  • The disc aerators on Pond 1 failed and were in the process of getting repaired.

Late 2023

  • Fencing around oxidation ponds was installed to improve safety and reduce rubbish entering ponds. Subsequent minor changes were made to deter birds and improve effectiveness.
  • Insurance negotiations were continuing.
  • In November, a report on the insurance claim for the November 2021 fire was provided to the Council. 
  • In December, aerators on Oxidation Pond 1 restarted to minimise odours.

March 2024

  • A report on the insurance process and odour mitigation efforts was provided to the Council.
  • Odour mitigation plan released. 
  • Odour mitigation work is underway. Various actions taken to improve plant performance and reduce odour. 
  • A new interactive map was created showing odour mitigation work. 

April 2024

  • Ponds switched to winter operation mode. Adjustments made to our operations to reduce odours when the biology changes as weather cools. 

May 2024

  • The Mayor and councillors approved a grant of $130,000 to resource a project with the goal of strengthening and sustaining effective and respectful relationships with Eastern Communities.
  • Completed work to cover inlet chambers to contain odour. 
  • Completed replacement of the bio-bed material in the odour control system. Old material reused as soil conditioner for native planting areas.

October 2024

  • Christchurch City Council and its insurers have agreed on a settlement in relation to the fire-damaged trickling filters at the Christchurch Wastewater Treatment Plant.
  • The Council also approved the replacement of the trickling filters with an activated sludge reactor.
  • This significant milestone allows us to move forward with building a resilient and more environmentally friendly wastewater treatment plant for our growing city.
  • Replacing the trickling filters with an activated sludge reactor will give the Council the opportunity to significantly reduce greenhouse gas emissions. Additionally, activated sludge reactors can deliver better environmental outcomes and have a smaller footprint that can be cost-effectively upsized to accommodate the needs of our growing population.
  • Design work is underway and it’s expected to take up to three years to complete the design and construction of the new activated sludge reactors.

November 2024

  • Demolition of the Christchurch Wastewater Treatment Plant’s fire-damaged trickling filters started.
  • Southern Demolition is doing the work, which is expected to take four to five months to complete. 
  • The demolition is the first stage of physical work to replace the filters with an activated sludge reactor, which will fix long-standing odour issues. 

April 2025

  • Demolition of the trickling filters was completed making way for construction to begin on the the new activated sludge reactor later this year. 

February 2026

  • The bulk earthwork started. This will be completed in four stages and is expected to finish around early April 2026 .  

April 2026

  • The installation of approximately 1400 stone columns will begin. This will take approximately four months to complete.
  • Installation of the reactor discharge pipe and the Returned Activated Sludge (RAS) line to commence. This is expected to be completed in late October.

June 2026

  • Work to install an engineered gravel platform will begin.  

August 2026

  • The engineered gravel platform work is expected to be completed.
  • Construction of the concrete structure for the new reactor will begin. This is a major part of the project and will take around one year to complete.

October 2026

  • Installation of the reactor discharge pipe and the Returned Activated Sludge (RAS) line expected to be completed.
  • The process to install the supply pipes from the existing pump station to the new plant will begin. Due to the pipes’ depth, sheet piling will be required for all underground pipes. This will be completed around March 2027.

2027

  • Construction of the blower hall will commence. This will house the blowers and electrical room. It will be constructed with precast concrete panels. This will take around four months to complete.
  • The installation of the supply pipes from the existing pump station to the new plant will finish up around March.
  • The blower hall is expected to be completed around April.
  • Installation of all the equipment and pipes required to operate the new plant will commence. This will start with the blower hall fit-out and then move on to the reactor.
  • Pumps and pipework in the existing pump station will be replaced to accommodate the new plant.
  • Construction of the concrete structure for the new reactor is expected to be completed around August 2027.

2028

  • Commissioning the new plant and replacing the remaining two pumps in the pump station will begin. This will enable us to shut down the existing temporary activated sludge plant and prepare to return the two clarifiers to their original purpose.
  • Removing the temporary plant and making any repairs to the clarifiers and other equipment to return them to normal duty.
  • The entire project is expected to be completed in late 2028.