Algal bloom has been detected at Walkers Lake, Avons Plains.
Warning signs were erected to alert residents and visitors of the algal bloom.
Blue-green algae has been known to cause skin and eye irritations, nausea, vomiting, muscle weakness or cramps after contact.
Northern Grampians Shire Council Mayor Murray Emerson said people needed to pay attention to the warning signs and steer clear of the lake for the time being.
"In the interest of community safety, we ask community members to keep themselves and their furry friends away from the water until the algal bloom has a chance to naturally dissipate," he said.
Algal blooms are a natural phenomenon in water bodies, making the water appear green as well as often appearing to have scum on the surface of the water.