Toxic algae have returned to moderate levels in the Hutt River.
The Greater Wellington Regional Council says if ingested the algae can kill livestock and dogs, and contact can cause vomiting, diarrhoea and skin irritations in humans.
The council has posted signs warning river users to be careful around the water and to keep dogs on a leash.
Toxic algae are affecting at least two other waterways in the North Island.
The Waipoua River near Masterton has high levels of toxic algae, and Lake Tutira in Hawke's Bay has been ruled off-limits altogether due to the discovery of potentially toxic cyanobacteria.
