THE Crocodile Hunter, Steve Irwin, is dead.

He was killed in a freak accident in Cairns, police sources said today.

It is understood he was killed by a stingray barb that went through his chest and reportedly into his heart .

He was swimming off the Low Isles at Port Douglas filming an underwater documentary when the tragedy occured.

The Queensland Ambulance Service (QAS) was called about 11am (AEST) and an emergency services helicopter was flown to the crew's boat on Batt Reef, off the coast near Cairns, with a doctor and emergency services paramedic on board.

Irwin had a puncture wound to the left side of his chest and was pronounced dead at the scene.

Irwin's body is being flown to Cairns.

One report today said his American-born wife Terri was trekking on Cradle Mountain in Tasmania and was yet to be told of her husband's death.

The Irwins have two children - a daughter, Bindi Sue Irwin, eight, and a three-year-old son, Robert (Bob) Clarence Irwin.

Steve Irwin - known worldwide as the Crocodile Hunter - is famous for his enthusiasm for wildlife and his catchcry "Crikey!"...

Marine experts say television personality and naturalist Steve Irwin had little chance of surviving after being struck in the chest by a stingray's barb.

Forty-four-year-old Irwin died yesterday morning in far north Queensland while filming for a marine documentary.

Sea World's marine science director Trevor Long says stingrays have killed a total of 17 people worldwide.

"All of those fatalities have occurred in the trunk of the body," he said.

"Many people, many fishermen have been stung by sting rays in the feet and the hands and it's not an unusual occurrence but it doesn't cause a great deal of problem.

"It's a venom - a protein-based venom - that can affect the heart."

Mr Long, who knew Irwin for years, says he was meticulous about safety in his documentaries.

"There's been a number of fatalities worldwide form stingray barbs," he said.

"The majority of those have been where the barb penetrated the trunk of the body.

"Steve had an intuitive sense about him. He knew animals and knew how to read animal behaviour. This would have been one of those freak unfortunate accidents."

Friends of the man known as the "Crocodile Hunter" say he died doing what he loved doing best.

Irwin had sailed his boat Croc One to the waters off Port Douglas to film a documentary on dangerous sea creatures.

His producer and close friend John Stainton says he was swimming above a stingray on Batt Reef when it suddenly pierced his heart with the barb on its tail.

Mr Stainton says it is the worst day of his life but at least his friend died doing what he loved.

"You think about all the documentaries that we've made and the dangerous situations that we have been in, you always think is this a day that may be his demise. I don't know. We'd been in some pretty close shaves," he said.

Police said a post-mortem examination was to be conducted on Irwin's body last night...