The Dunedin City Council is trying to find out what caused a bad smell which upset residents near Tahuna Wastewater treatment plant yesterday.
Several people contacted the council soon after noon complaining about a smell they said was so appalling they had to cover their noses to try to block it out.
It was noticeable until about 12.30 but had gone by about 1.30pm.
While he was unsure where the waste originated, council 3 Waters group manager Tom Dyer believed it could have been "a particularly smelly" batch of industrial waste.
"It doesn't happen often. It was a bit of a nasty surprise," he said.
All the waste brought in by tankers was contained until it was discharged and Dyer was unsure why there was a problem yesterday.
"We normally go through a couple of processes to prevent anything like this happening."
There had been no previous similar incidents and 3 Waters was talking to the tanker company and trying to find out what happened.
"We're working with them to see what went wrong so we can try to avoid any similar situations in the future," Dyer said.
The upgraded Tahuna treatment plant treats about 30 million litres of wastewater each day from around 80,000 residents and from industry.