The Wanganui District Council has been unable to identify the industry - or industries - responsible for a significant weekend dumping of sulphides that has left Wanganui's wastewater treatment plant struggling to cope.
And Mayor Annette Main has warned residents to expect the associated odour to linger for some time.
She said the amount of waste discharged breached the existing trade-waste consent limits held by any of the city's major industries.
"The problem increased on Friday, with sulphides being received between 6.15am and 5pm. We then experienced exceedingly high levels of sulphides from 2.27am on Saturday and this continued throughout the night and morning," Ms Main said. "We haven't been able to identify which industry - or it may be more than one industry - is responsible for this weekend's discharges."
Ms Main said the news would be really distressing for residents and visitors, given the smell.
"The stench has been appalling at Kaitoke and Lake Wiritoa on Saturday morning with the westerly wind.
If the wind turns to the east, then Castlecliff, Gonville and the central city will be affected."
She said there was no "quick fix" for the odour problem.
"And I'm sorry to say that it's going to continue. The only alternatives to discharging to the treatment plant would be to discharge directly into the Whanganui River or the ocean. No-one in our community wants that."
Ms Main said shutting down the major industries was also not an option.
"They are an important part of our economy and many families rely on them for their income," she said.
"Again, I apologise to the community for the terrible odour problems. They are beyond the Wanganui District Council's control and it is obvious that we will need to have a major review of our industrial trade-waste systems in the new year."