Wanganui's wastewater had to be sent into the sea yesterday because of excessive levels of hydrogen sulphide.
Mayor Annette Main said the high levels caused strong odours in parts of the city and required the temporary diversion of wastewater directly to the sea.
"Early Friday morning, Castlecliff was hit by a hydrogen sulphide odour. We don't believe this was directly from the wastewater treatment plant but was contributed to by the failure of systems at Tasman Tanning's Tod St site.
"The failure has resulted in bypassing of their sulphide treatment area, meaning some sulphides have been going directly into the sewerage system," she said.
"Our monitoring equipment at Beach Rd pumping station shows that since Thursday evening, Beach Rd has been receiving high hydrogen sulphide levels at regular intervals and this continued throughout Friday. Higher levels are to be expected at the height of the season.
"We've been fortunate that cooler, windy weather conditions [yesterday] have assisted in containing the smell and preventing it having a wider effect. Some of the hydrogen sulphide reached the treatment plant before we began diversion to sea, so the ponds have been affected and this could result in increased odours.
"To help reduce this, we started applying lime to the ponds from 7am on Friday and have significantly increased the atomiser neutralising fence around the ponds.
"From 11.15am the wastewater was being diverted straight to sea, bypassing the treatment plant. This will continue until the excessive levels of hydrogen sulphide reduce."
It was hoped to stop the diversion last night or today.
Ms Main called it "absolutely a decision of last resort".
The situation had been complicated by yesterday's fire.
"A number of hazardous materials had to be washed away by the Fire Service and some of this has entered the sewerage system. This means it is too dangerous to use hydrogen peroxide treatment as the wastewater goes through Beach Rd pumping station and we've stopped the addition of any treatments at Beach Rd until Monday morning."