Green Lake has been closed for recreational use after potentially toxic blue-green algae was observed this week.
Horsham Rural City Council has advised people who have come into contact with water from the lake to wash affected skin with clean water immediately.
Samples of water from the lake are undergoing laboratory tests to confirm the species of algae.
Council is also advising people not to use the affected water for cooking, drinking, washing or showering.
"Boiling the affected water will not make it safe to use," the warning read.
Council staff will continue to monitor the lake for the presence of toxic algae, and provide updates as required.
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