The Canterbury District Health Board is urging people to avoid several local waterways that have toxic algal blooms.
The board says Lake Ellesmere, the Ashley River above the State Highway 1 bridge and the Rutherford Crescent Reserve pond in Hanmer Springs should be avoided.
There are increased concentrations of cyanobacteria in the waterways, which can cause gastroenteritis and is particularly harmful to dogs, the DHB says.