Lake Burrumbeet has been closed due to a blue-green algal outbreak.
The City of Ballarat said warning signage advising the public of the lake's closure to all activity would go up soon.
The lake would be shut "until further notice", with weekly visual inspections to observe any visible blooms, scum formation and evidence of dead fish, the council said.
The council said water testing would be conducted fortnightly.
Blue green algae is common during warmer than expected temperatures, with a major outbreak also forcing the lake's closure last March.
Water affected by blue green algae can be unsuitable for drinking, recreational activities such as swimming and for fishing and agricultural uses.
The contamination can result in taste and odour issues in water supplies, while some species can produce toxins harmful to humans and animals.
The council said as a result swimming, water sports, wading and fishing were now banned at the site.
Extracting water for stock or domestic use was also banned and no stock or pets should be allowed to make contact with or drink the water, the council said.
Lake Learmonth also felt the effects of last week's heatwave with piles of dead fish washing up on the banks Friday.
Several lake users blamed low oxygen levels in the shallow, heated water.
Lake Learmonth Advisory Committee chairman Fon Ryan said the return to dry conditions was concerning for the recreational use of the lake.