Another Canterbury waterway has been contaminated by toxic algae.

The Canterbury District Health Board has issued a public health warning for Lake Pegasus.

Medical officer of health Dr Cheryl Brunton says people and animals, particularly dogs, should avoid the area until the warning's been lifted.

She says the algae looks like dark brown or black mats and exposure may cause rashes, nausea, stomach cramps and numbness.

Brunton says people should see their doctor right away if they show any symptoms.

There is also a warning in place for the Selwyn River near Glentunnel.