Irwin, 44, was in Cairns, Queensland filming a segment for the series Ocean's Deadliest when the incident occurred at around midday local time (1am UK).
Close friend John Stainton told reporters: "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. We got him back within a couple of minutes to Croc 1, which is Steve's research vessel, and by 12 o'clock when the emergency crew arrived they pronounced him dead."
He added: "The world has lost a great wildlife icon, a passionate conservationist and one of the proudest dads on the planet. 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!"
Irwin leaves behind a wife and two children - Bindi Sue, 8, and Bob, 2.