Temperatures soared across the Wimmera and Grampians at the weekend, with some parts of the region reaching 44 degrees on Saturday.
While many took to the water to cool down, they had to avoid popular Horsham waterhole Green Lake, which has been closed due to blue-green algae outbreak.
Horsham Rural City Council technical services director John Martin said samples of water from the lake showed potentially toxic levels of blue-green algae.
“Tests have confirmed what we thought – that the levels are potentially toxic and unfortunately, the lake will need to remain closed to swimming and boating for the time being,” he said.
Mr Martin said council staff would continue to monitor the lake.
“Unfortunately it’s a difficult thing to control,” he said.
“Hopefully the algae will dissipate as quickly as possible.”
Mr Martin urged people to avoid the lake’s water.
“People who come into contact with this water should wash affected skin immediately in clean water,” he said.
“People are also advised not to use the affected water for cooking, drinking, washing or showering.
“Boiling the affected water will not make it safe to use.”
Mr Martin said the council would let people know when the algae returned to a safe level.