Hawke's Bay Regional Council says that black mat algae has been identified in the Waipawa River.

Dog walkers are advised to be very cautious and keep dogs under control at all times by the river as the mats are believed to be toxic.

The District Health Board has posted warning signs along the river to warn dog walkers and hikers.

Phormidium occurs naturally in shallow rivers and looks like a blackish mat that forms on rocks and with the drop in river levels it has resulted in phormidium being exposed at Waipawa.