Canterbury District Health Board is warning people to avoid the Waipara River after potentially toxic algae was found there.
Medical Officer of Health Alistair Humphrey says the river at Teviotdale Bridge be avoided until the warning is lifted.
Dr Humphrey says exposure to the algae may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
He says no one should drink water from the river at any time. Boiling the water does not remove the toxin.