Hurunui River users in North Canterbury have today been warned of a potentially toxic blue-green algae.

The Canterbury District Health Board has warned people that they and animals, particularly dogs, should avoid the water in the Hurunui River at State Highway 1.

Dr Alistair Humphrey, Canterbury Medical Officer of Health, said the cyanobacteria looks like dark brown to black mats and can produce toxins harmful to people and animals.

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

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People should remain out of the waterways until the warnings have been lifted.