Wannon Water has advised that toxic levels of Blue Green Algae have been detected in Lake Bullen Merri following testing last week.
Blue Green Algae may cause harm to people and animals.
People should avoid any contact with the water including swimming, fishing and boating activities.
Warning signs have been activated at north and south beaches.
Wannon Water will continue to monitor algae levels and advise when the lake is safe to use again.