Canterbury people are being warned to stay away from the Rakahuri/Ashley River due to dangerous toxic algae in the water.

People and animals should not go towards the river while a health warning is in place, according to the community and public health division of the Canterbury District Health Board.

The DHB issued the warning after discovery of detaching mats of potentially toxic blue-green algae (benthic cyanobacteria) in the river.

Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.

The algae look like dark brown to black mats and can produce toxins harmful to people and animals, Canterbury Medical Officer of Health Dr Ramon Pink said.

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"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."

The algae occur naturally but can increase rapidly during warmer months.

It often has a strong musty smell and algal toxin concentrations can vary over short periods with changing environmental conditions.

Although high river levels will remove the algal bloom, detached mats can accumulate along the shore and increase the risk of exposure to toxins.

Reticulated town water supplies are currently safe but no one should drink water from the river at any time.

Pets that show signs of illness after coming into contact with algal mats should be taken to a vet immediately.

Environment Canterbury is monitoring the sites and the public will be advised of any changes in water quality.

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Warnings to stay out of Te Waihora/Lake Ellesmere, Selwyn River at Whitecliffs Domain, Selwyn River at Glentunnel, and Ashley River Rangiora/Loburn Bridge are still in place.