People have been warned to keep themselves, pets and stock away from the Waikaia River and the upper reaches of the Mataura River after a sighting of the potentially toxic benthic cyanobacteria (blue/green or brown/black algae).
The algae was seen in the Northern Southland rivers on Wednesday during regular monitoring by Environment Southland, surface water quality scientist Roger Hodson said.
Cyanobacteria algal mats had been observed floating downstream which meant the algae could also be present in the Mataura River, but that had yet to be confirmed, he said.
The algae had been seen as far downstream as Gore in previous years and it was possible it was there again now, he said.
Mr Hodgson said cyanobacteria occurred naturally and was not necessarily toxic.
Samples had been taken but it would be the middle of next week before he knew whether the Northern Southland algae was toxic.
However, Environment Southland and Public Health South had ''acted immediately'' to issue a precautionary warning, he said.
''We want to alert people to the heightened risk.''
People were advised to avoid contact with the Waikaia River from Piano Flat downstream, including the main stem of the Mataura River, until further notice.
