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.
Early voting for Secret Harbour by-election begins Monday
Welfare concerns for missing Swedish national last seen in Warnbro
Wanted man last seen in Golden Bay
Passenger critically injured in West Pinjarra crash
Unregistered painter fined for requesting hefty deposit from Singleton homeowner and not doing the job
Police hunt driver following dangerous Baldivis pursuit
Welfare concerns for boy last seen at Port Kennedy skate park
Rubbish truck catches fire in Baldivis
Driver injured in bus crash in Rockingham