Toxins in Henley Lake have forced Masterton District Council to warn dog walkers not to let their animals off leads.
Parks and recreation services manager Grant Hathaway said the warning follows the discovery of toxins related to the decay of algae bloom.
"A health warning was issued earlier this week and signs have been put up at the lake after localised algae blooms were identified. This is a cautionary approach taken by council to ensure that lake users are not exposed to these toxins," he said.
"Exposure to cyanobacteria may cause symptoms such as skin rashes, nausea, tummy upset and tingling and numbness around the mouth or tips of fingers. If you or your pets experience health symptoms after contact with contaminated water, contact your doctor or vet immediately," Mr Hathaway said.
He described the toxin as causing "a thick surface scum" covering localised areas of the lake shoreline.