The 26-year-old was repeatedly bitten on his left leg by what his friends thought was either a bull or tiger shark.
The attack occurred while the man was diving near Hinchinbrook Island, off the Queensland coast.
Following the attack the man's friends took him by boat to the mainland town of Cardwell where paramedics stemmed the bleeding and stabilised his condition.
He was then flown by helicopter to Cairns Base Hospital, some 90 miles to the north.
Neil Noble, Queensland Ambulance Service (QAS) Supervisor described the attack as a "very close call", adding that the victim "lost consciousness and lost a significant amount of blood".
Mr Noble added that one of the man's friends was a former Navy officer who used his medical training to apply a tourniquet to the injured man which "had a significant impact on the survival of this gentleman, without which he may not have survived this incident".
Bull sharks have an average size of between 7ft 4in and 7ft 9in (2.25m-2.4m), while tiger sharks can grow to sizes of between 10ft 8in and 13ft 11in (3.25m-4.25m).
In January a 55-year-old man was severely injured when he was attacked by a 13ft (4m) bull shark while diving near the Great Barrier Reef.
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