Canterbury swimmers are urged to beware - with toxic blue-green algae found in the Selwyn River

The DHB has issued a public health warning for the Glentunnel area.

It says people and animals - particularly dogs - should avoid the area until the warning's been lifted.

The algae looks like dark brown or black mats - and exposure may cause rashes, nausea, stomach cramps and numbness.

People are being urged to see their doctor right away if they exhibit any symptoms after going near the area.