A potentially toxic type of algae has been spotted in North Otago's Kauru River.

Otago Regional Council staff spotted Phormidium - a type of algae that forms thick dark brown or black mats on riverbeds - in the river at Ewings, west of Oamaru.

Environmental resource scientist Hugo Borges said contact with it could cause rashes, nausea and numbness around the mouth and fingers for humans.

"While most of the warm and dry conditions that cause these algae blooms are behind us, there is still a risk of encountering Phormidium at certain spots, and it pays to be especially cautious with pets," he said.

Dogs are especially vulnerable to the velvety-textured algae as they seem to be drawn to its musty smell and are inclined to eat it.

"The algal mats can detach from the river bed and accumulate along the water's edge, making them more accessible to dogs. Dogs seem to like the smell of Phormidium and it can cause severe poisoning, even death if they consume it."

Mr Borges said people should avoid the water, and contact their vet if they have concerns for their animals. The most common symptoms in dogs include fast breathing, salivation, twitching, muscle tremors, paralysis, lethargy, convulsions, or frothing at the mouth shortly after being near the water.