Christchurch City Council said on Wednesday the finding has been backed up by its monitoring stations beyond the boundary of the Bromley plant.
An investigation identified the odour was coming from the primary settlement tanks, which process wastewater before it enters the temporary activated sludge treatment plant, said a city council spokesperson.
Council staff are currently in the process of making changes to reduce the odour from these tanks.
"This is likely to take a couple of days to resolve. We’ve also engaged a consultant who specialises in odour monitoring to assist staff in identifying and responding to odour sources," the spokesperson said.
The foul smell was exacerbated when a fire badly damaged the Bromley plant's two trickling filters in November 2021, affecting the quality of effluent flowing into the system and causing an unpleasant stench to spread across eastern parts of the city.
"There is better news from the oxidation ponds. Our monitoring shows the ponds’ recovery over the holiday period has been maintained, with the majority of the ponds green, and only two still set at their orange setting on the tracker," the spokesperson said.
"There are currently 13 of 16 aerators operational, while our contractor works on repairing and returning the other three aerators to service as soon as possible.
"We appreciate some of this information is disappointing, and our staff are doing all they can to reduce the odour as quickly as possible."
