High levels of toxic algae have been found in the Hurunui River at State Highway 1 and people have been warned to avoid the area.
Canterbury District Health Board says animals, particularly dogs, should also be kept away because of the high levels of the potentially toxic benthic cyanobacteria Phormidium.
Medical officer of health Dr Alistair Humphrey says the cyanobacteria (algae) look like dark brown to black mats and can produce toxins harmful to people and animals.
He says exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
People who experience the symptoms should see their doctor immediately and pets showing illness after coming in contact with cyanobacteria mats should be taken to a vet.