The Department of Environment and Climate Change issued an advisory after blue-green algae was found in Lake Ainslie yesterday.
They say people should not swim or bathe in the lake, and keep children and pets out of the water.
Anyone who takes their water from the lake, or has a shallow well next to the lake, could have contaminated drinking water.
Blue-green algae are microscopic bacteria that live in surface water like lakes, ponds, rivers and streams; it can be dangerous to people and pets.