Three Queenslanders are being hailed as lifesavers after plucking their mate from the water and desperately applying a tourniquet to his seriously wounded leg after he was bitten by a shark.

The officer in charge of the Cardwell ambulance station, Martin Taylor, has praised the quick actions of the trio after their friend was attacked while spearfishing about 10.20am on Saturday.

Taylor said the 26-year-old was bitten when he jumped into the water at the popular fishing spot of Eva Rock near Hinchinbrook Island, off the coast of Cardwell in far north Queensland.

"From what I've been told he was in the water, he's been hit, they've heard a noise and they've dragged him in as soon as they've realised what's gone on," Taylor said.

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He said the three friends pulled him into their boat and one of them, an ex-military serviceman with first aid training, tied a tourniquet high up on the man's thigh to stop the massive blood loss.

"The initial actions by the three friends have definitely saved this gentleman's life," Taylor said.

He said ambulance officers treated the man at the scene before taking him to the local sporting field to await a rescue helicopter, which then transported him to a Cairns hospital.

The man, who has been awake and talking, remains in a stable but critical condition.

Cairns ambulance officer Neil Noble said they believed his chances of survival were high.

"We believe that he may have a very positive outcome," he said.

"He's been stabilised by the resuscitation team in the emergency department and he's been rushed up to theatre for emergency surgery."

There are unconfirmed reports the shark was either a bull shark or a tiger shark.