Beaches in South Australia, including Waitpinga and Parsons near Adelaide, have been closed following the appearance of dead fish and unusual white foam washing ashore.
Surfers reported symptoms such as eye irritation, sore throats, and coughing after contact with the water.
Authorities suspect a microalgae bloom, possibly triggered by recent warm and dry weather, as the cause.
Water samples have been collected for analysis, with results expected by the weekend.