Health warnings have been erected at two Canterbury sites following the detection of the potentially toxic alga, cyanobacteria.
The dark brown algal mats have been found in the Selwyn/Waikirikiri rivers at the Whitecliffs Domain and the Ashley/Rakahuri rivers near State Highway 1.
People and animals, particularly dogs, should avoid these areas.
The Canterbury Medical Officer of Health, Dr Alistair Humphrey, said reticulated town water supplies are safe, but no one should drink the water from the river at any time.
Meanwhile, the Canterbury DHB has lifted the algal bloom health warning issued at Lake Forsyth/Wairewa in October this year.