An Australian surfer is in a critical condition after he lost his leg in a shark attack at a Western Australian beach.

Ben Gerring, 29, a fly-in-fly-out heavy machinery operator, was surfing at Falcon Bay Beach in Mandurah when the attack occurred, local media reported.

The Fisheries Department said a great white shark measuring at least three metres was likely to be responsible for the attack, given the nature of the victim's injuries, the Nine Network reported.

The City of Mandurah has closed Falcon Beach and Gerring has been taken to Royal Perth Hospital, according to local media.