A toxic algae has been found in the Waikaia River and may be present in other Southland waterways.

Environment Southland director of science and information Graham Sevicke-Jones advised its latest monitoring round found elevated levels of the toxic benthic cyanobacteria in the Waikaia, at Waikaia.

It had been found in the Mataura and recent warmth meant it was probably present in other areas.

''People should take extra care when choosing a swimming spot or letting dogs off leads along rivers.''

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The algae can produce toxins harmful to people and animals if swallowed, or through contact with skin. People and animals (dogs in particular) should avoid contact with the water and any mats that had formed at the waters' edge.

Benthic cyanobacteria can be recognised as a green/brown slime on rocks, or dark brown/black mats at the water's edge.