Potentially toxic algae has been found in the Ashburton, Waimakariri and Ashley Rivers.
Canterbury Medical Officer of Health Dr Ramon Pink says the algae loosk like dark brown or black mats and can produce toxins harmful to humans and animals.
Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
He says no one should drink the water from the river, and people should know boiling the water from the river does not remove the toxin.
Reticulated town water supplies are currently safe.