Greater Wellington Regional Council and health officials are warning residents and animals could be at risk from toxic algae in parts of the Hutt River.
Regional environmental senior scientist Summer Greenfield says heavy rain combined with warm weather have produced rapid growth of cyanobacteria in the river downstream from Birchville.
The council says people should watch for the algae described as dark brown or black coloured mats, growing on rocks in the river bed.
It says the mats can be fatal to livestock and dogs and a risk to residents. Contact with the algae may cause vomiting, diarrhoea and skin irritations.
The council has posted warning signs at key access points along the Hutt River and says there may also be moderate risk of cyanobacteria in Waikaenae and Waipoua Rivers.