Authorities said he received a barb to the chest at about 11 am local time on Batt Reef, off Port Douglas. Crew called for treatment from an air ambulance, but Irwin was already dead aboard research vessel Croc One when it arrived.
Irwin's producer John Stainton said: "He came over the top of the stingray and the barb went up into his chest and put a hole in his heart."
Attending physician Dr Ed Loughlin confirmed nothing could be done to save Irwin. He said: "It became clear fairly soon that he had non-survivable injuries. He had a penetrating injury to the left front of his chest. He had lost his pulse and wasn't breathing."
Fellow documentary maker Ben Cropp said he had spoken to Irwin's cameraman. He told news.com.au: "In this case he [Irwin] was swimming alongside a bull ray, a big black ray and the cameraman would have been in front, filming him. Steve got probably maybe a bit too close to the ray, and with the cameraman in front, the ray must have felt sort of cornered. It baulked but didn't spook and go racing away, which would have been fine.
"It went into a defensive mode, stopped, turned around and lashed out with its tail which has a considerable spike on it. Unfortunately, Steve was directly in its path and he took a fatal wound."
There are more than 400 species of stingray. Their barbs can be over a foot long and carry a paralysing toxin. Despite this, deaths are very rare, as most are sustained to the legs or feet when stingrays are trodden on in the shallows.
Tributes to Irwin and his work have been quick to come. PM John Howard said: "I am quite shocked and distressed at Steve Irwin's sudden, untimely and freakish death. He was a wonderful character. He was a passionate environmentalist. He brought joy and entertainment and excitement to millions of people."
"I really do feel Australia has lost a wonderful and colourful son," he added.
Stainton said he had always feared their 20 year relationship would end in tragedy. He said: "Nothing would ever scare Steve or would worry him. He didn't have a fear of death at all."
Irwin leaves a wife and two children.