A surfer who lost his leg during a shark attack south of Perth yesterday is fighting for his life in hospital, as authorities hunt the animal responsible.

Ben Gerring, 29, lost his leg above the knee and was in a critical condition in the Royal Perth Hospital this morning, after he was attacked just before 4pm yesterday off Falcon Beach, about 80km from Perth.

The Fisheries Department believes a great white shark measuring at least three metres was responsible for the attack, given the nature of Mr Gerring's injuries.

They laid drumlines in the vicinity of the attack about 7am, which trapped a large shark this afternoon.

The shark was seen thrashing around the lines and then being towed further out to sea by Fisheries staff.

Ben Gerring (left) and Fisheries staff patrolling the water. (9NEWS)

Witnesses to the attack said Mr Gerring had screamed for help before being dragged under the water.

His friend Brian Williams said Mr Gerring had just paddled further out into the ocean to catch a bigger wave when "all hell broke loose".

Mr Williams said he was getting ready to go out to surf himself when a group of men came rushing back to shore with Mr Gerring.

"They jumped to work straight away, it was pretty horrific to see someone you know in that predicament," he told ABC radio.

"There are images there that are going to be pretty hard to get out of your mind for a while ... I've spoken to a lot of the guys that are struggling at the moment with the scenes they saw."

Another local surfer, Gary Bing, said he was told how the shark launched out of the water.

"He was on the beach at about 4.10pm and they were giving him CPR as the police arrived, the ambos got there about five minutes later then they kept doing CPR, the chopper arrived and they just kept pumping his chest. It didn't look good," he told 6PR radio.

"The board has got some teeth marks in it, but it has basically been snapped like something has landed on it, the snap is so clean it's unbelievable."

Shark policy expert and University of Sydney lecturer Christopher Neff has said trying to kill the shark is pointless "political theatre".

The WA government dumped its controversial baited drumline trial in 2014, after the environmental watchdog recommended against extending it, but is allowed to implement its "imminent threat" policy for any shark larger than three metres that poses a risk to public safety.

Mr Neff said it was pointless to kill the shark suspected of the attack.

"The catch and kill policy is not a shark bite prevention policy - it is political theatre," he said.

"And it's a shame because it basically takes a terrible situation that happened to people, like this gentleman, and makes it worse by suggesting the solution to the problem is to kill an individual shark.

"That's not the solution to the problem."

Do you think sharks that attack people should be killed? Yes 3447 No 8678

Dr Neff said baited drumlines made the beach more dangerous as they brought in more sharks.

Fisheries Department regional manager Tony Cappelluti has also defended its shark warning system, saying the morning shark sighting had been published on its Shark Smart website and on Surf Life Saving WA's Twitter account.

Mr Campbell said it was up to people to check those websites before entering the water.

"The sighting was 1.8km off the coast, there was no reason to act ... it is very hard for us to predict what's going to happen," he said.