People are urged not to swim at Owhiro Bay in Wellington because of sewage contamination.
Water-quality monitoring has found enterococci bacteria at a level likely to be a health hazard, Wellington City Council says.
People could suffer diarrhoea, stomach cramps and vomiting if exposed to contaminated water, while cuts could become infected.
The council says the source of the contamination is not obvious.
Water samples are being taken at various points in the Owhiro Stream and local sewer mains are being checked.