Harris is a cave diver with over 30 years of experience. Harris' cave diving experiences include leading a team of Australian divers to record depths of in 2011 and 2012 whilst searching for the source of New Zealand's Pearse River, the mission was filmed for National Geographic. In 2011, Harris was requested by the South Australian Police to participate in the recovery of the body of his close friend Agnes Milowka, who had died whilst exploring a cave near Tantanoola in the south east of South Australia. In 2009 Harris was awarded the "Outstanding Achievement" award at the Australian technical diving conference Oztek, to mark his exceptional contributions to cave diving exploration, in 2017 he was awarded the "Australasian Technical Diver of the Year" at Oztek.
In June 2018 Harris was about to depart on a cave diving holiday to the Nullabor Plain when he and dive partner Craig Challen were requested by the Thai government, on the advice of British cave diving experts attempting to rescue twelve Thai children and their soccer coach who were trapped in the Tham Luang Nang Non cave system, to provide assistance with the rescue efforts. Harris' efforts throughout the rescue have been described as essential; he conducted a medical assessment of all of the trapped boys and it was on his advice that the authorities reversed their initial strategy of rescuing the strongest boys first, followed by the weakest. Harris decided the order in which the boys would be extracted and advised on the method to be used. To allow the rescue to occur Harris administered a mild sedative to the boys, ensuring they did not panic during the rescue. Harris and Challen were the last members of the rescue team out of caves following the rescue. On 24 July 2018 Harris, along with Challen, was awarded the Star of Courage and Medal of the Order of Australia by the Governor-General of Australia. On 7 September 2018 the King of Thailand appointed Harris as a Knight Grand Cross of the Most Admirable Order of the Direkgunabhorn. On Australia Day 2019, Harris was announced as the joint 2019 Australian of the Year with Challen. On the 5 November 2019 Doctors Harris and Challen, released a book "Against All Odds" chronicling their participation in the rescue of the boys from the Tham Luang Cave. In the book and in radio interviews promoting the book they corrected some of the inaccuracies in the media regarding the rescue. He stated that he did not pick the order that the boys would leave the cave, and that the boys decided that amongst themselves based on the distance each boy lived from the cave. He also said the boys were fully unconscious as they were carried through the flooded cave, as he had given each boy two intramuscular injections in the thigh: Ketamine to put them to sleep, and Atropine to suppress saliva production to stop choking.
