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.
Community invited to name crane towers building new Mandurah Hospital
Hundreds of volunteers remove 525kgs of rubbish from Mandurah's waterways
Port Kennedy man fined $4.2k for failing to feed horse
Rockingham General Hospital patients face the longest waits for elective surgery
Australian-first drone technology to monitor emergencies, crimes in Baldivis, Kwinana, Rockingham
Teen accused of ordering firebomb attack on Baldivis tobacco shop
City of Mandurah sees sharp rise in animal cruelty reports
Geoff Valentine returns to Peel Thunder as senior coach
Golden Bay woman charged with lacing colleague's curry lunch with snail poison