Irwin was at Batt Reef, off the remote coast of northeastern Queensland state, shooting a segment for a series called "Ocean's Deadliest" when he swam too close to one of the animals, which have a poisonous barb on their tails, his friend and colleague John Stainton said.
"He came on top of the stingray and the stingray's barb went up and into his chest and put a hole into his heart," said Stainton, who was on board Irwin's boat at the time.
"It's just absolutely unbelievable," Nigel Marven, a well-known wildlife specialist who was a longtime friend of Irwin's, said on The Early Show. "To be killed by a big crocodile or bitten by a snake, you'd have believed it. But a stingray? There's only been three cases in Australia of deaths by stingrays in the last 100 years. Seventeen worldwide. So it's a tremendous freak accident."
Crew members aboard the boat, Croc One, called emergency services in the nearest city, Cairns, and administered CPR as they rushed the boat to nearby Low Isle to meet a rescue helicopter. Medical staff pronounced Irwin dead when they arrived a short time later, Stainton said.
2Irwin was famous for his enthusiasm for wildlife and his catchword "Crikey!" in his television program "Crocodile Hunter." First broadcast in Australia in 1992, the program was picked up by the Discovery network, catapulting Irwin to international celebrity.
He rode his image into a feature film, 2002's "The Crocodile Hunters: Collision Course" and developed the wildlife park that his parents opened, Australia Zoo, into a major tourist attraction.
"The world has lost a great wildlife icon, a passionate conservationist and one of the proudest dads on the planet," Stainton told reporters in Cairns. "He died doing what he loved best and left this world in a happy and peaceful state of mind. He would have said, 'Crocs Rule!"'
Prime Minister John Howard, who hand-picked Irwin to attend a gala barbecue to honor President Bush when he visited in 2003, said he was "shocked and distressed at Steve Irwin's sudden, untimely and freakish death."
"It's a huge loss to Australia," Howard told reporters. "He was a wonderful character. He was a passionate environmentalist. He brought joy and entertainment and excitement to millions of people."