A reduction in odour levels has been recorded at the Christchurch Wastewater Treatment Plant, with council officials saying warmer weather has helped improve pond conditions over recent days.

Christchurch City Council said monitoring showed Pond 2B, located just south of Breezes Road, had been downgraded from a high odour risk to a possible odour status. As a result, hydrogen peroxide dosing has been reduced for now.

A council spokesperson said staff had kept a close watch on pond health over the past few days and the improvement was encouraging.

“The recent run of fine weather has helped improve conditions, particularly in Pond 2B, which has allowed us to lower its odour risk rating and scale back peroxide dosing,” the spokesperson said.

The spokesperson said council staff would continue working through the long weekend and remained on track to install a new aerator for Pond 2B early next week.

“Unless odour levels worsen unexpectedly, our next public update will be issued on Monday. Work is continuing and the installation of the new aerator is still scheduled for early next week,” the spokesperson said.

Council has also updated its odour status map to reflect the downgrade of Pond 2B from red to orange. The map shows current odour risk levels and the time of the most recent update and is available on the council website.

Health officials have reiterated advice for residents affected by the ongoing stench. Public Health Medical Specialist Dr Imogen Evans said exposure to hydrogen sulphide odours from the treatment plant could cause nausea, headaches, eye and throat irritation, skin irritation, sleep disturbance, and worsening asthma symptoms at relatively low concentrations.

She said even when physical symptoms were not present, prolonged exposure to unpleasant odours could still negatively affect mental wellbeing.

Dr Evans said hydrogen sulphide does not build up in the body and any health effects were expected to resolve once odour levels returned to normal, with long term health effects considered highly unlikely.

Residents experiencing health issues linked to the odour were advised to seek medical advice from their GP or healthcare provider, or contact Healthline on 0800 611 116 at any time. In an emergency, people should call 111.

The council said further updates would continue to be published online as work progressed to stabilise the treatment plant and reduce odour impacts on nearby communities.