Mullaloo beach is temporarily closed due to an algae bloom.
A department of biodiversity, conservation and attractions spokesperson confirmed the bloom is Trichodesmium.
"This bloom has most likely formed due to the hot and calm conditions Perth has experienced over the recent days," the spokesperson said.
"It is not known to be harmful to humans however it can cause minor skin irritations.
"It is recommended to not swim/walk through the bloom and keep pets away.
"If you do come in to contact, rinse with freshwater immediately."
Tests are underway to confirm the cause of the bloom which locals fear is related to a wastewater treatment plant.
Kim Allen from the Mullaloo Surf Life Saving Club told Kate Leaver, on ABC Perth Breakfast with Mark Gibson, the bloom has been around for a few days.
The City of Joondalup was contacted for comment.