A man has lost his leg and is fighting for his life after a shark attack at Falcon Beach in Mandurah on Tuesday afternoon. 

Fly-in, fly-out worker Ben Gerring was surfing with friends at a popular surf break around 4pm when he was mauled by an unknown shark. 

The 29-year-old man, who works for Fortescue Metals Group, was rushed from Surfbreak Lookout to Peel Health Campus by ambulance around 5pm.

A friend on Facebook said, "we are all praying for you Benny, you gotta make it brother," while another said "you're a fighter my brother, knew you would pull through keep punching my man."

At 6.30pm, the RAC rescue helicopter was preparing to transfer Mr Gerring to Royal Perth Hospital.

Witness reports claimed Mr Gerring was waiting to catch a wave with around 15 others in the water nearby when he was attacked from behind by a shark. 

A Falcon Bay Beach Cafe worker said two men rushed in asking for ice for a man's leg that had been bitten off around 4pm. 

The City of Mandurah has closed the beach.

A member of the public, on Tuesday morning, reported a 3.5 metre great white shark about 1.8 kilometres off-shore at Pyramids Beach - around four kilometres south of Falcon. 

- with Mandurah Mail