Health warnings have been issued for the Waihao River in South Canterbury and Lake Pegasus in Waimakariri District after potentially toxic algae was discovered in them.
South Canterbury District Health Board says monitoring has identified moderate cover of the cyanobacteria in the Waihao River near Bradshaws Road.
Potentially toxic blue-green planktonic cyanobacteria was found in Lake Pegasus.
Community and Public Health has issued the health warnings and signage will be erected at the Waihao River site until monitoring indicates the water is safe.
There are also other access points along the Waihao River that may have cyanobacteria present and people are being advised to check for the presence of the algae and avoid contact.
Humans and animals, particularly dogs, should avoid the areas until the health warning has been lifted.
Canterbury Medical Officer of Health Dr Alistair Humphrey says the algal mats can produce toxins harmful to humans and animals.
"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips," Dr Humphrey said.
"If you experience any of these symptoms, visit your doctor immediately, also let your doctor know if you've had contact with dark brown/black algal mats or water in this area."
No one should drink the water from the river at any time, as boiling the water from the river does not remove the toxin, he said.
Animals should be taken to a vet immediately if they come into contact with the contaminated rivers.
Environment Canterbury is monitoring the Waihao River and the public will be advised of any changes in water quality.
Lake Pegasus is being monitored on a weekly basis.
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