Residents, especially dog owners, are being advised to avoid contact with the Waikaia River in Southland because of toxic algae.
Environment Southland's latest monitoring showed elevated levels of benthic cyanobacteria, which can be potentially fatal to dogs.
The council's director of science and information, Graham Sevicke-Jones, said that the algae may be present in other areas along the river.
It has already been found in the Mataura River, for which Waikaia River is the main tributary.
He said it could also be present in other rivers in the region, and people should be cautious when choosing swimming spots or letting dogs off their leads.
Benthic cyanobacteria is a green or brown slime found on rocks, or dark brown or black mats at the water's edge.