Warning: Stay away from toxic river
A toxin dangerous to people and animals has prompted a health warning for the Waihao River.
High levels of potentially toxic dark brown algae, also known as phormidium, have been found in the river near Bradshaw's Bridge.
Phormidium is found in rivers throughout New Zealand. Under the right flow conditions it forms thick black mats.
The algae, recently formed in the Waihao River, has started detaching and accumulating along the edge of the river.
People are warned to avoid the area and to keep animals, particularly dogs, away until the health warning has been lifted by Community and Public Health.
South Canterbury medical officer of health Dr Daniel Williams said anyone exposed to the toxin could suffer a range of symptoms.
"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.
"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."
He said no-one should drink the water from the river at any time, as boiling it would not remove the toxin. Animals exposed to the toxin should be taken to the vet immediately.
Environment Canterbury is monitoring the sites and will inform the public of any changes to the water quality.
The Waihao River, at The Black Hole, was also affected by the algae last summer, along with areas of the Opihi and Pareora rivers. Heavy rain helped reduce the toxin to safe levels.