People are being warned to steer clear of parts of the Ruamahanga River in the Wairarapa because of toxic algae.
Greater Wellington Regional Council says the black or brown algae forms on the river bed and can quickly kill dogs if eaten.
It can also cause vomiting, diarrhoea and other allergy-type symptoms in people who come in contact with it.
River users should especially avoid the popular picnicking spot at Kokotau Bridge, east of Carterton.
The council's senior environmental scientist, Summer Greenfield, says the warm weather is behind the increase, and the risk is likely to remain until there is significant rainfall to flush the algae away.