Steve Irwin caught his first venomous snake, a common brown, when he was six years old, and by nine he was helping pull problem crocodiles off Queensland boat ramps. He never stopped. Irwin, the zookeeper and conservationist the world knew as the Crocodile Hunter, died at 44 on 4 September 2006 at Batt Reef off Port Douglas in far north Queensland, struck in the chest by a stingray barb while snorkelling for a documentary shoot.

The pursuit was handed to him early. His parents, Bob and Lyn Irwin, moved the family from Victoria up to Queensland's Sunshine Coast and opened the Beerwah Reptile Park in 1970. By 1980 it had been renamed the Queensland Reptile and Fauna Park, and Steve had grown up inside it, among the animals his parents collected and cared for. Australia Zoo records that he took over running the place on 4 October 1991, at 29.

Long before the cameras, there were the crocodiles. Irwin spent months at a time in remote far north Queensland volunteering for the Queensland government's East Coast Crocodile Management Program, catching problem animals that had become a danger to people and either relocating them or housing them at the family's park. He caught more than 100 that way, and the capture and handling methods he worked out in those years were taken up by crocodile handlers elsewhere. In a 2003 interview with ABC's Australian Story, he described the pull in terms with no science in them at all: "I think wildlife is just like a magnet, and it's something that I can't help."

Two days after he took over the park, an American visitor walked in and stood listening to a young man talk about crocodiles. Terri Raines ran a wildlife rehabilitation organisation called Cougar Country back in Oregon, taking in cougars, bears and bobcats. They married on 4 June 1992 in Eugene, her home town, and went straight to work instead of on a honeymoon. Filming for The Crocodile Hunter began that same year, and across it and the documentary series that followed the two of them would film more than 300 episodes together.

What made the show land was that Irwin was not a presenter visiting wildlife. He was a zookeeper who happened to have a camera pointed at him, in the khaki shirt and shorts that became his signature. Broadcast around the world, the series was watched by more than 500 million people, by Australian Geographic's count. Wes Mannion, his best friend, later told the crowd at Irwin's memorial about their first meeting, when Mannion was 14: "Steve's enthusiasm was instantaneous and infectious."

The money went back into the animals. He and Terri renamed the family park Australia Zoo in 1998 and ran it on a phrase of his own, Conservation Through Exciting Education. That was the year his daughter Bindi was born, named after one of his favourite crocodiles; his son Robert arrived in 2003. In 2002 he and Terri founded the charity Wildlife Warriors so that other people could take part in protecting injured, threatened and endangered wildlife, and built it on a line of his that it still prints: "If we can teach people about wildlife, then we can save it." The charity now funds work on black rhinoceros, Sumatran tigers, koalas, cheetahs, whale sharks, manta rays and grey nurse sharks. The Australia Zoo Wildlife Hospital opened in 2004, named for his mother, Lyn. In 2001 the Australian Government had given him the Centenary Medal for service to global conservation and to Australian tourism.

The least visible part of the work was the science. In 1990 Irwin and his father found an unrecorded freshwater turtle in the Burdekin River system and sent photographs to the Queensland Museum; the species was later named Elseya irwini, Irwin's turtle, after them. It is a strange animal, able to draw oxygen straight out of the water through its cloaca, and it is now known from little more than a 25 kilometre stretch of the Bowen River inland from Mackay. Australia Zoo and Wildlife Warriors have since established an insurance population of the species, with the aim of breeding the turtles and releasing them in the future. Irwin also helped develop a way of following wild crocodiles by fitting them with transmitters, a technique the University of Queensland still uses in the crocodile research it runs with Australia Zoo. Professor Craig Franklin of UQ said Irwin had made "a significant contribution to crocodile research in Australia and in particular helping to develop new methods to track these often wary animals through remote sensing technology". The university was preparing to appoint him an adjunct professor when he died. Terri accepted the Steve Irwin Adjunct Professorship from its School of Integrative Biology in November 2007 and said that "the work that he did with crocodiles was something that's never been done before".

On 20 September 2006 thousands of people filled the Crocoseum, the crocodile arena at his zoo, for a public memorial broadcast internationally. Prime Minister John Howard told them Irwin "had that quality of being genuine, of being authentic". Howard also said of him: "He taught our children in particular to love and respect all creatures great and small." The actor Russell Crowe sent a recorded tribute: "Your passing has suspended reality for all of us." His father Bob said: "The animals lost the best friend they ever had and so have I." Then Bindi read from a sheet of paper. She began: "My daddy was my hero and was always there for me when I needed him." She finished, as ABC News reported, with a promise: "Daddy made this place his whole life, now it's our turn to help daddy."

Two decades after his death that promise is being kept. Australia Zoo covers more than 700 acres at Beerwah with a staff of over 500, and its wildlife hospital takes in around 10,000 native Australian animals a year. The Steve Irwin Wildlife Reserve, 135,000 hectares of Cape York about 60 kilometres northeast of Weipa spanning 35 distinct ecosystems, has carried one of the world's longest-running crocodile studies since 2007. Every 15 November, Steve Irwin Day, people pull on khaki and raise money for Wildlife Warriors. In 2009 the Queensland Museum malacologist John Stanisic named a mountain-top land snail Crikey steveirwini because its creamy yellow, orange-brown and chocolate shell came out khaki. In 2015 the Queensland Government made him the first posthumous Queensland Great. And the small reptile park his parents opened at Beerwah in 1970 is still run by his family: Terri, Bindi and Robert.