Wanganui District Council has been issued with an abatement notice by Horizons Regional Council following ongoing smells from the city's wastewater treatment plant.
Horizons group manager strategy and regulation Nic Peet said the notice was a legal directive requiring the council to cease causing an objectionable odour beyond the boundary of its plant within the next four weeks.
It follows a formal warning issued in December 2012.
Dr Peet said that, while Horizons recognised solving issues might be difficult, the odour was objectionable and its ongoing impact on the people of Wanganui meant it must be resolved.
"Industrial processes like those occurring at the wastewater treatment plant will always produce an odour. It becomes an issue when an objectionable odour extends beyond the property boundary, affecting other residents and businesses, as is clearly the case in Wanganui.
"We know the district council is taking steps to address the issue and is keeping Horizons staff appraised of progress. It is a complex problem but the district council is in breach of regional rules around air quality and, as the regulator, we need to act accordingly," Dr Peet said.
He said if the city council was unable to get rid of the smell within four weeks, Horizons would look at why that was.
"There are a range of further enforcement actions we can take, but we certainly won't be hitting them with an automatic fine."
He declined to say what those further actions might be.
Dr Peet said the council had identified that unusually large amounts of trade waste entering its plant had resulted in the recent widespread smell.
"We understand that Wanganui District Council will be ensuring that the industries discharging industrial waste to the plant do not exceed the conditions set in their consents with the district council.
"However, the plant has had a number of problems since it opened, which mean that it has not being functioning as it should.
"This raises the fundamental question of whether the plant, as it is currently designed, can cope with even normal consented amounts of industrial and domestic wastewater and, if not, what needs to be done to fix it."