People in Wellington are being urged to look out for toxic algae in rivers because they can grow rapidly in hot weather between weekly monitoring.
Toxic algae levels have triggered warnings from the Wellington Regional Council - advising people not to swim in the Hutt River between Moonshine Bridge and Fairway Drive Bridge.
The Pakuratahi River from State Highway 2 to its junction with the Hutt River in Kaitoke Regional Park is also out of bounds.
A Senior Environmental Scientist at Greater Wellington Regional Council, Mark Heath, said sites in Waikanae, Otaki, and Wairarapa had shown early signs of toxic algae.
He said warm weather could have a fast effect on levels so people should know what they look like.
"So we encourage people to go to our website, Is it safe to swim? to learn more about the toxic algae and especially what it looks like because it can increase very rapidly in a short space of time."
Dr Heath said children should be kept away from water in these sites and dogs should be kept on a lead.