For the second time in just under two years, GWMWater has confirmed elevated levels of blue-green algae in Lake Toolondo.
On Friday, a spokeswoman said: "Visitors can still enjoy recreational activities such as boating, bushwalking, sightseeing and other activities at the lake that do not involve direct water contact.
"Warning signs have been positioned at major recreational areas around the lake and will remain in place until blue-green algae levels drop below the threshold considered safe for recreational use."
The spokeswoman said it was not possible to predict for how long the levels would be high.
She said blue-green algae occurred naturally and could be dangerous to humans and animals.
Characteristic signs of algae contact are skin rashes or itchiness, sore eyes, ears and nose; or if swallowed, nausea or vomiting.
Pet owners should prevent pets from drinking or having direct contact with blue-green algae affected water.
Similar warnings remain in place for Green Lake and Lake Charlegrark.
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