Horizons Regional Council is advising people to steer clear of the Mangatainoka River near the Tui Brewery after the discovery of a large amount of blue/green algae in the area.
Toxins produced by the algae, also known as cyanobacteria, have been known to cause animal deaths and can cause skin irritation in humans. Dogs and young children are particularly susceptible.
Horizons science manager Jon Roygard says it is not yet known whether this particular strain of cyanobacteria is toxic, however testing at the site in 2011 showed the algae to be toxic in most cases.
"The slimy, black algae can bloom very quickly in warmer weather and will appear as a shiny black mat on the rocks. Warning signs have been erected and if you see it, it is best to stay away," Dr Roygard said.
"If you are concerned your dog may have ingested the algae, it is recommended you contact your veterinarian immediately."
The Mangatainoka site near the Tui Brewery is one of 10 sites tested weekly by council. This testing is being carried out to help Horizons understand the causes of cyanobacteria to assist in management and prevention.