Cantabrians looking to cool off with a swim today are being urged not to do it in Te Waihora, Lake Ellesmere, because there's a toxic algal bloom.

People and animals should avoid drinking, touching or swimming in the water.

The Community and Public Health division of Canterbury District Health Board has issued a health warning after high concentrations of the blue-green algae planktonic cyanobacteria were found in the lake.

The algae occur naturally but can increase rapidly during warmer months.

Not all cyanobacterial blooms are visible to the naked eye and toxins can persist after the blooms disappear.

CDHB spokesman Dr Ramon Pink said boiling the water does nothing to remove the toxin.

Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips.