The 59-year was airlifted from Britomart Reef to Townsville Hospital where he is being treated for a serious wound to his leg, with paramedics describing his injuries as "catastrophic".
Police spokesman Andrew Cary told Australian broadcaster 9 News: "He was on a vessel with a friend, spear-fishing at the time when the attack happened."
David Humphreys, a doctor with LifeFlight Australia, added: "There was obviously evidence of a very significant shark bite to the patient's upper thigh, pelvic region.
A man is fighting for his life after a shark attack in Queensland
The man is taken to the waiting air ambulance
The obvious concern was catastrophic bleeding which we did our best to get control of
"He remained in a critical condition after our interventions and he's now undergoing emergency surgery here in the hospital.
"The obvious concern was catastrophic bleeding which we did our best to get control of.
"But knowing the definitive care is surgery for this patient, we wanted to stabilise him as far as we could."
(More to follow)
The attack happened at Britomart Reef in Queensland
Police spokesman Andrew Cary
Lifeflight doctor David Humphreys