The malfunction of the Wanganui wastewater treatment plant is believed to have been caused by a local industry dumping a large amount of solid waste.
The Wanganui District Council partially shut down the plant after an excess amount of solid waste was dumped into the Beach Rd pumping station over the weekend.
Residents in the Kaitoke and Marybank area complained of the "horrendous" smell that caused sore throats and tainted clothes. By Tuesday the smell had spread across the city, although some of this smell was believed to have come from industrial sites on Heads Rd.
Wanganui District Councillor Ray Stevens said analysis of Beach Road pumping station information showed the system received about 56 tonnes of solid waste per day for the preceding few days. Usually it receives around 10 tonnes.
Mr Stevens yesterday confirmed the council was investigating the source of the dumping.
"One of our local industries has disappointingly ignored its resource consent," he said.
Mr Stevens said it was a short-sighted action that affected the whole of Wanganui. He said it also set back the council's bio-augmentation programme.
"If anyone is having a problem with their consent they should come and talk to us, and we may be able to accommodate them," he said.
Council infrastructure manager Mark Hughes said the high waste levels caused failures in the UV plant at the wastewater treatment plant, but that would be functioning again within the next day.
"The type of waste solids introduced caused a thick flotsam of mainly fatty products to form on the surface of the main pond. This thick, heavy scum is being manually removed using boat, boom and sucker truck. Left, it would quickly become rancid and cause further but different odour problems," Mr Hughes said.
A range of solutions was being used, focused mainly on reducing odour. Mr Hughes said odours from the plant had reduced and complaints about the smell were expected to stop as the treatment takes effect.
Mayor Annette Main said she acknowledged the odour had been offensive and widespread.
"I can assure people that district council staff are making the resolving of this a priority. This is not a situation any of us wishes to continue," Ms Main said.