Swimmers are being warned to not enter the water at The Pond in Safety Bay.
Recent water quality tests have produced "elevated results", but the City of Rockingham has not revealed what was found in the tests.
The City has put up warning signs around the area which state swimming in the water may cause "ill effects to humans and animals".
"The signs will be removed once further water results show the water quality has returned to safe swimming levels," the City of Rockingham wrote on their Facebook.
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