Dog owners in Hawke's Bay are being urged to take care when walking along the Tukituki and Waipawa rivers, where toxic algae has appeared.
Black mat algae, called Phormidium, occurs naturally in many Hawke's Bay rivers and is not linked to poor water quality.
However, it has the potential to cause serious illness in people and dogs, and can release toxins into the water.
Hawke's Bay Regional Council ecology scientist Dr Gary Rushworth said currently there was an added risk to dogs because river levels were low.
"Dogs are attracted to the musty odour of the exposed or detached toxic mats that have washed ashore and dried, and may eat them," he said.
"It's important that dog owners are aware of what black mat algae look like. It can also be brown or grey when it has dried.
"Always treat the black growth on rocks in the water and the drying mats as toxic."
Dr Rushworth said those exercising their dogs should swim or walk them somewhere else.
The Hawke's Bay District Health Board has posted warning signs where there is known to be a high risk from the toxic algae.