Police say the man was competing with the Mollymook Longboarders at Golf Course Reef when the incident occurred.

He had participated in four heats and was taking part in the final at about 12.30pm when he was noticed lying face down in the water approximately 100 metres from shore and 20 metres from rocks.

The fellow competitor, a 14-year-old boy, assisted by another competitor, an off-duty police officer, dragged the unconscious man onto a surfboard and commenced basic first aid as they paddled him back to shore.

On the beach they performed CPR with the assistance of a third competitor - an off-duty paramedic - while they waited for NSW ambulance service personnel to arrive.

Police say the man was in full cardiac arrest and was revived by defibrillator.

He was stabilised before being airlifted by NSW Ambulance helicopter to St George Hospital in a critical condition.

Police say it is unknown how the man came to be unconscious in the water however it is believed he may have been there for approximately two minutes.

Shoalhaven Local Area Command duty officer, Acting Inspector Ray Stynes, said the outcome could have been very different if it hadn't of been for the quick actions of those on the scene.

"The teenager and the off-duty officer did an extraordinary job getting the man onto a surfboard and back to the beach while also rendering first aid," Acting Inspector Stynes said.

"We believe the man might have been unconscious in the water for about two minutes before being rescued," he said.

"If he wasn't found and brought to shore quickly, where the off-duty paramedic immediately rendered first aid, the outcome could have been far worse," he said.

It is expected the three fellow competitors will be nominated for bravery awards.