Nelson City Council is demanding answers from a contractor about the stench from a sewage treatment pond which has wafted across the city for four days.
People from northern Nelson to Stoke, several kilometres to the south, have been complaining about it since Friday.
A school near the Wakapuaka ponds north of the city has stopped its outdoor classes, while the smell has forced some residents to keep windows closed.
Clifton Terrace School principal Rob Wemyss says the smell has been a recurring problem since March last year.
The city council says it's not the same issue as last year - for which the council was fined $1000 - and it is baffled by the latest odour.
Spokesperson Alec Louverdis says the council spent $9 million upgrading the ponds three years ago.
Council staff met representatives of contracting company UGL at the ponds on Tuesday, and are contacting other councils and treatment pond experts around the country looking for solutions.
Residents had a reprieve from the smell on Tuesday, with Mr Louverdis saying a move to seed the ponds with healthy algae appears to be working.
Overcast conditions and an offshore wind also helped to keep the odour away from homes on Tuesday.