A surfer has lost a leg in a shark attack south of Mandurah in Western Australia. The attack happened at Gearies Beach in the Mandurah suburb of Falcon an hour before sunset.

The surfer, 29-year-old Ben Gerring, was taken to hospital in a serious condition.

Mandurah Mail reporter Nathan Hondros spoke to the ABC not long after the attack.

"The gentleman was on the beach by the time we got down here," Hondros said. "Ambulance crews were trying to resuscitate him...it was just quite desperate and heart wrenching scenes.

"He had lost a leg from what looked like above the knee, so it was quite a savage attack by the looks of it."

Mr Hondros said it was unclear what type of shark was behind the attack. "We had a talk to some of the surfers who were out there in the water with him, but look, they weren't in the mood to talk while their mate is in serious trouble," he said.

Gearies is a popular surfing spot for surfers and swimmer. Mandurah Board Riders president Brian Williams said he knew Gerring well and had just paddled further out into the ocean to catch a bigger wave when "all hell broke loose".

"He was right out the back looking to get a big wave, one of the set waves that had been coming through during the day," Mr Williams said. "He'd sort of paddled out the back, sort of past the pack slightly. And next thing all hell broke loose and they were trying to bring him in.

"His board was broken in half, they found the tail part of his board."

Two surfers with longboards paddled out to bring Gerring in.

“On any given day there’s probably around about 50-60 people in the water,” Mr Williams said. “I’m not sure but I think this would be the first time it’s ever actually happened here.”

Between 2010 and 2014 WA's south west had a cluster of shark attacks and fatalities. The incidents had lessened until this afternoon's attack.