Dog owners are being warned to keep their pets out of the water at Dunedin's Ross Creek Reservoir following the discovery of toxic algae.
Cyanobacteria, a blue-green algae which can produce toxins harmful to humans and animals if swallowed, has been found in the water.
The algae can also be harmful through contact with skin.
The Dunedin City Council said warning signs would be in place at the popular dog walking spot until monitoring showed the cyanobacteria was no longer a danger.
DCC general manager infrastructure and development Simon Drew described the risk as low.
"However, we’re taking a precautionary approach."
The reservoir water was lowered in August to carry out maintenance work on the valve tower and tunnel inside the dam.
The low level means the water is heating up more than usual, which could be contributing to the algal bloom, the council said in a statement.
The reservoir will remain at its current level for the summer until the maintenance work is completed.
