The Hutt City Council is urging people to stay out of a swimming spot on the Wainuiomata River due to high bacteria levels.
The contaminated area is a section of the river at Richard Prouse Park.
The council warns that diarrhoea, stomach cramps and vomiting can be caused by drinking the water.
Divisional manager of regulatory services Geoff Stuart says warning signs have been put up.
Bacteria levels are expected to return to normal in a few days but testing of the river will continue, he says.