The man was surfing at Gracetown around 8 a.m. when he was attacked, St. John Ambulance spokesman Dennis Bertoldo said.
Surf photographer Peter Jovic told the ABC news he had witnessed the attack.
"There was a lot more thrashing around after that, it was pretty hard to see what was going on".
He said paramedics on the ground reported that first aid from the man's friends was crucial in the moments after the attack, as they used a leg rope to stem blood flow.
The man, in his 30s, was flown to Royal Perth hospital with leg injuries, but was conscious, St John Ambulance said. Chris Boyd, 35, was killed at nearby Umbies surf break in 2013.
Surf Life Saving WA had noted several sightings of sharks in the area, including a four-metre shark off Lefthanders, near where the attack happened, this morning.
The Shark Smart website says the carcass could make nearby waters unsafe until it's removed.
Gracetown beaches, including North Point, Big Rock and Lefthanders, have been closed.
An alert has been issued warning beach-goers that the carcass could draw sharks closer to shore.
With all eyes on the nearby contest at Margaret River, WSL officials chose to put the event on hold-and considering Mick Fanning's close call at the J-Bay Open in 2015, it seems like a good call.
Organisers confirmed the incident off the coast of Gracetown - 260 kilometres (162 miles) south of Western Australia's capital Perth - which prompted an one-hour suspension in the competition.
He said the incident emphasised the need to do more, including using SMART drumlines, rather than the Labor government's subsidised shark deterrent devices.
Former champion surfer Mick Fanning escaped unscathed when a great white attacked his board as he waited to catch a wave.
"The current policies are simply not adequate".