An incident management team is responding to reports of a brown sludge-like substance in waters off the coast of Port Fairy.
The Department of Environment, Land Water and Planning (DELWP) has established the team which is collecting water samples from in and around The Passage.
The samples will be analysed to identify any alteration to water quality.
Drone monitoring will also take place.
The Standard is aware of one surfer who says he became sick after surfing at The Passage.
A statement released by DELWP on Thursday afternoon said an update would be provided to the community once results were available.
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Members of the public are advised to avoid direct contact with water in and around The Passage until further notice.
Anyone who experiences illness after coming into contact with the water at this location should seek medical advice.
DELWP incident controller Daniel Sullivan said the collection and analysis of water samples would be taken across Thursday and Friday.
"Further decision-making will take place when the test results are available," he said.
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