Marlborough District Council is telling swimmers not to go into the sea at Picton after routine testing turned up unsafe levels of bacteria in the water.
High levels of such contamination are reasonably common in Picton after periods of heavy rain, when the town's sewage system is prone to overflowing into the waterways.
Picton has received more than 100mm of rain since Christmas.
The council says warning signs are up and water samples are being taken daily.
