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The attack happened at the Farm Break surf spot near to Bunker Bay at about 2.15pm on Friday, and emergency services quickly rushed to the scene.
The shark bit the man's leg, before he whacked it with his surfboard and other surfers managed to get to him and intervene.
The other surfers then managed to get the injured man - 28-year-old called Phil Mummert - to shore.
One witness explained: "Someone yelled out that it was a shark, I turned around and I seen a fella being attacked.
"Three guys that were close to him they started to paddle towards him...he was already off his board trying to push the board into the shark.
"Those three guys got him onto the beach... everyone came together, there were guys off calling triple-0 on their phones straight away."
The witness continued: "He's just sitting on his board and the next thing you know we just heard someone go 'shark!'
"I turned around and I've just seen this white [shark] kind of breach. It just obliterated his board, it was just floating there like icicles.
"I got over to Phil and just like, 'alright mate, jump on my board', because I was riding a log, like a nine-foot board.
"So, [I] kinda just chucked him up on that and then another bloke, I think his name was Liam, paddled over and was shouting at everyone, you know, 'come on, come help', because it's better to group up.
"Just from there on trying to get him into shore and keep him out of shock... A couple of the gashes were 150mm long.
"He held composure completely, he didn't really even blink."
The surfer was taken via helicopter to Bunbury Hospital. His partner Mish Wright shared an update on Facebook saying: "Just want to let everyone know that Phil is doing okay! He was bitten by a shark at Bunker Bay this afternoon and got a few chunks taken out of his leg!
"I honestly don't know how a person can see a total stranger getting attacked by a 4m great white shark and swim TOWARDS to save him so we are beyond grateful to everyone that helped save Phil's life!"
An investigation into the incident has been started by the Department of Primary Industries and Regional Development, and the beach has also been closed down.
A DPIRD statement reads: "A male surfer received injuries after being bitten by a reported 4m white shark at approximately 2.15pm"
"Department of Primary Industries and Regional Development is working with local authorities to coordinate responses.
"DPIRD Fisheries officers are onsite conducting beach patrols and the RAC helicopter is conducting aerial surveillance in the area.
"City of Busselton rangers have closed off the beach."
The future of the shark, if it is found, has not been decided.
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