Witnesses to a shark attack on the mid north coast of New South Wales thought the victim was having a heart attack until they were told to get out of the water.

Black Head Beach surf life saver David Quin-Livian was bitten to the bone by the shark.

He made it to shore and has since been airlifted to hospital.

Officials are investigating what type of shark is behind the attack, while beaches in the area have been closed for at least the next 24 hours.