Wellington Regional Council is warning dog owners to keep their pets on a lead near some parts of the Hutt River, after the detection of a large amount of toxic algae.
The black algal mats were found in the Boulcott area between the Melling and Kennedy Good Bridges.
The council said the algal mats were widespread, and are becoming exposed on the river's edge as the water level drops.
If ingested, the algae can kill livestock and dogs, and in humans can cause vomiting, diarrhoea and skin irritations.
The council's senior environmental scientist, Summer Greenfield said algae levels were low at the river's main swimming sites.