People in Canterbury are being advised to avoid two rivers in the region after high levels of a potentially toxic algae were detected.
Canterbury Regional Council has found phormidium, which can be harmful to humans and animals, in the Ashley River in Rakahuri near State Highway 1 and the Waipara River at Stringers Bridge.
The Canterbury medical officer of health says people should not drink contaminated water from these rivers, even if boiled.
Alistair Humphrey says exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips in humans.
Symptoms in dogs include muscle tremors, twitching, paralysis and convulsions.