"I was in the area surfing when the alarm went off and we heard there was a shark in the water," Iva Busic told The West Australian.
"Everyone ran out of the water. I am pretty shocked."
Another witness told the ABC police are "looking for a body," however police have yet to confirm their condition.
"Hopefully they can find it," said the WA resident.
The attack prompted a huge emergency response; police and paramedics flooded the normally idyllic locations.
At least three St John's Ambulance crews were at the beach, and police helicopters hovered overhead the shoreline.
Police rescue boats took to the water, while officers on foot patrolled the closed beaches.
Shark activity was reported near the beaches at 10.14am by Dorsal Shark Reports.
The Department of Primary Industries and Regional Development confirmed they were investigating the attack.
"DPIRD received reports of a serious shark bite incident at approximately 10.06am on the 6 November 2021," it said in a statement.
"DPIRD is working with local authorities and further information will be provided as it becomes available."
This attack comes months after a man was bitten in waters off the state's north.
The Perth man, aged in his 30s, was attacked near Varanus Island, off the coast of Onslow in Western Australia's north in July.
A St John Ambulance spokeswoman said at the time, the man was attacked in the evening and he suffered significant but not life-threatening injuries.
She said he had "a few bites" to his body and first aid was provided on-board the vessel.
That included providing pain relief, as well as cleaning and bandaging his wounds.
"I think they did a pretty good job — they controlled the bleeding," she said.
