Christchurch Wastewater Treatment Plant update

Friday 10 April 

Pond health and odour

Overall pond health remains much the same. One noticeable shift is that water temperatures across the ponds are slowly cooling, which is a clear sign that the system is transitioning into winter mode.

Seasonal changes like this can sometimes be confusing, particularly because water quality, algae levels and biological activity naturally behave differently at different times of the year.

As temperatures drop, the biological processes in the ponds slow down, which can change how the ponds look and perform – even when everything is operating as expected. Check out the explainer item lower in the newsletter to find out more about how seasonal changes can impact odour. 

Aerator planning underway

Our teams have been working hard since the Council decision last week to increase the amount of aeration in the ponds. We'll provide an update soon, once we've confirmed the next steps and have a more detailed timeline. 

What to expect this weekend 

As we head into the weekend, pond health remains stable, with no changes to the current odour status.

An explainer: Seasonal changes and odour 

Odour at the Christchurch Wastewater Treatment Plant doesn’t stay the same all year. It follows a seasonal cycle driven by temperature, sunlight and biology in the ponds. Summer and winter are generally the most stable periods. Odour is more likely during the change between seasons.

Summer: Usually low odour 

In summer, the ponds usually work very well. Warm temperatures and long sunny days help algae grow. The algae produce oxygen, which helps "good" bacteria break down wastewater without creating strong smells.

Why it smells less in summer

• Lots of sunlight = lots of oxygen
• Oxygen stops smelly gases from forming
• Warm weather helps treatment happen faster

Transition to autumn: Odour risk increases 

As daylight shortens and temperatures cool, the ponds have to adjust. Algae produce less oxygen and the biological balance shifts. During this change, odour can sometimes be more noticeable.

Why smells can happen

• Less sunlight = less oxygen
• The system is rebalancing
• Short‑term changes can lead to odour

Winter: Usually low odour 

In winter, everything in the ponds slows down. Colder temperatures reduce biological activity, which means fewer odour‑causing gases are produced and released.

Why it smells less in winter 

• Cold weather slows bacteria
• Less gas is produced 
• Odour-causing gases are less noticeable

Transition to spring: Odour risk increases

Spring brings warmer days and more sunlight. Biological activity increases quickly as algae return. This rapid change can briefly upset the balance in the ponds and increase the risk of odour.

Why smells can happen 

• Sudden biological "wake-up"
• Rapid changes in oxygen levels 
• Short-term instability in the system

Stay updated

We want everyone to be able to stay updated on what's happening at the Christchurch Wastewater Treatment Plant, and we regularly post new information on ccc.govt.nz/wastewaterfire 

You're welcome to send this update to your networks. If you know someone in the community who doesn’t have email or internet access, please print this newsletter off and pass it to them.

If you know of anyone who would like to receive this e-newsletter they can subscribe online. 

If you have any questions, or would like further information on the project, please call us on 03 941 8999 or 0800 800 169 or email wastewater@ccc.govt.nz  

Thanks for your patience and understanding. We’ll be in touch again soon.