A lake near Christchurch commonly used for recreation has been declared unsafe.
There has been a sudden increase in algal bloom and associated toxins in Lake Forsyth, also known as Lake Wairewa.
The Canterbury District Health Board says if people come into contact with the water, adverse reactions could develop, such as irritation, rashes and blistering around the mouth, nose and eyes.
The board's medical officer of health, Dr Alistair Humphrey, says people should also not eat shellfish taken from the lake.
