The Bribie Island man told his rescuers he was on the main deck of his 10-metre fibreglass power catamaran when it suddenly upturned, despite the calm conditions, about 25 nautical miles off the Sunshine Coast early this morning.

With the help of a passing fellow fisherman, the man was able to summon the Mooloolaba Volunteer Coast Guard and was eventually airlifted to hospital by the AGL Action Rescue helicopter.

"He said he doesn't really know what happened it just flipped, he thinks maybe it was a whale," AGL Action Rescue crewman Jerm Cutelli said.

"He said he was trapped under the vessel in an air pocket for a while and then got the courage to dive down and swim free. A nearby fisherman came to help and they called the coast guard."

The coast guard volunteers called the rescue helicopter after the fisherman complained of chest pain.

The fisherman was winched aboard the helicopter shortly after 11am and flown to Nambour General Hospital in a stable condition.

Mr Cutelli said recent training exercises with the coast guard had prepared the helicopter crew for today's rescue.

"That training meant today's real-life rescue was uncomplicated, everyone knew what they were doing, we were in and out in only a few minutes," he said.