A witness, who was paddle boarding in the water when the attack happened, pulled the victim to shore.
Lawyer Mark Hickey said the victim appeared to have been swimming at the time and it is thought he was doing the popular swim from The Pass to Byron Bay.
He saw what he thought was a turtle and some seaweed in the water about 20 metres away but then realised it was blood and a two metre shark circling.
"The shark came back to him and had another go. I didn't know it was a person - but when I realised I ran out and waded to the bank and grabbed him and did CPR but it was too late.
"I brought him back in and worked on him on the beach," he told the the Sydney Daily Telegraph.
Emergency services attend Clarkes Beach at Byron Bay in far northern New South Wales. Photo / AAP
Mr Hickey performed CPR for 20-30 minutes on the victim before rescuers arrived.
Holidaygoer Christian Campano, 35, told Daily Mail Australia everyone on the beach was shocked.
"(It was) pretty heavy, we're just in shock," Mr Campano said. "It's all starting to hit home."
He and his wife, Michelle, had gone for dip in the ocean when they were pulled from the water after the attack.
The couple headed up the beach and saw the victim receiving CPR.
Eventually, he saw police officers place a blanket over the man's head. "He just looked still," Mr Campano said.
Police have closed all Byron Bay beaches - from Tallows in the south to Belongil Beach in the north - to the public for the next 24 hours.
NSW government officers are being brought in to determine the type of shark involved in the attack.
A spokesman for NSW Ambulance said: "A rescue helicopter and paramedics were dispatched to the scene. A doctor immediately treated the man on the beach but unfortunately despite clinical intervention including CPR the man has been pronounced deceased on the beach as a result of his fatal injuries."
Read more at Daily Mail Australia
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