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THE Crocodile Hunter, Steve Irwin, is dead. It is believed he was killed by a stingray barb that went through his chest.

The Essendon-born wildlife enthusiast was swimming off the Low Isles at Port Douglas filming an underwater documentary when it happened.

The Queensland Ambulance Service (QAS) said a call was received about 11am (AEST) today and an emergency services helicopter was flown to a boat on Batt Reef, off Port Douglas, with a doctor and emergency services paramedic on board.

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

Irwin's American-born wife Terri learned of her husband's death while trekking on Cradle Mountain in Tasmania.

It is understood Irwin, 44, was killed around noon, Eastern Australian time.

A source said Irwin was already dead when his body was brought on to the island.

His body was being flown to Cairns Hospital in North Queensland for formal identification.

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

The father of two's Crocodile Hunter program was first broadcast in 1992 and has been shown around the world on cable network Discovery.

He has also starred in movies and has developed the Australia Zoo wildlife park, north of Brisbane, which was started by his parents Bob and Lyn Irwin.

Tributes have already started pouring in for the larger-than-life character.

Foreign Minister Alexander Downer, who used a photograph of his family at Australia Zoo for his official Christmas card last year, hailed Irwin for his work in promoting Australia.

Irwin was heavily involved in last year's "G'Day LA" campaign.

"The minister knew him, was fond of him and was very, very appreciative of all the work he'd done to promote Australia overseas," a spokesman said.

Mr Irwin had close links with Prime Minister John Howard and was a guest at The Lodge during a function for US President George W Bush in 2003.

A Tourism Queensland spokeswoman said the death was shocking and paid tribute to Irwin's "enormous contribution" to his adopted state.

Louise Yates said it was impossible to quantify how much Mr Irwin had meant to the Queensland tourism industry.

"I don't think we could even estimate how much he brought us through his personality and his profile and his enthusiasm about Queensland," she said.

"It would be difficult to estimate how much he was worth. And it would be difficult to underestimate."

She said Irwin had been a larger-than-life ambassador.

"It's not just what he brought but what he took with him when he travelled, his passion."

Australia Zoo, on southeast Queensland's Sunshine Coast, employs more than 500 people and attracts thousands of visitors every day.

But Ms Yates said it would be "unfair and unjust" to put a dollar value on Irwin's worth to the state, because of how much he had given.