A health warning has been issued for Lake Pegasus, which sits in the middle of a Canterbury housing development.
The Canterbury District Health Board is warning people to stay clear of the lake in Pegasus Town, due to the discovery of a potentially toxic blue-green algae.
Medical officer of health Dr Ramon Pink says the algal bloom can produce harmful toxins and animal owners - particularly those with dogs - should avoid the lake.
"Exposure may cause skin rashes, nausea, stomach cramps, tingling and numbness around the mouth and fingertips."
No one should drink from the water at any time, and boiling the water does not remove the toxin, Dr Pink says.
The algal bloom will be monitored on a weekly basis.