Potentially toxic bacteria has been found in the Waihao River near Waimate.
South Canterbury District Health Board is warning that people and animals should stay away from the river after the cyanobacteria algae was spotted in it near Bradshaw Road.
Warning signs have been placed near the river and the Canterbury Regional Council is monitoring the water for any changes.
The Canterbury Medical Officer of Health Dr Alistair Humphrey said exposure to the algae can cause skin rashes, nausea, stomach cramps, tingling and numbness.
Dr Humphrey said boiling the water does not remove the toxin.