Swimmers have been warned to avoid an area on the Taupo waterfront after the second wastewater spill into Lake Taupo in a week.
A "fat berg" produced by fat and wet wipes caused wastewater to escape from a manhole cover on Sunday night, leading to a spill at the end of Hawai St, Taupo District Council says.
The affected area is immediately around the outlet and does not affect the Two Mile Bay Sailing Centre.
A temporary health warning is in place advising against swimming in the area while water samples are tested. The test results are expected on Tuesday.
When wastewater escapes from a manhole, the resulting spill enters the stormwater system through nearby grates and gravity takes it into the lake.
Operational services group manager Kevin Strongman says contractors were at the site soon after the alarm was raised.
He says the council is as frustrated as the community at what happened.
"Our teams are constantly working to monitor the system and improve it, but we really need the public to play their part," he said.
"Wet wipes and nappy liners should not end up in the wastewater system - they should be disposed of in the rubbish."
Last week, swimmers were told to avoid Acacia Bay, where an unknown quantity of material had entered the lake at the stormwater outlet off Wily Terrace.
That incident was caused by human error after contractors doing routine checks failed to restart a pump.