Regan Harrison's first international success was a silver medal, behind Simon Cowley, in the 100 metres breaststroke at the 1999 Sydney Pan Pacs. At Sydney 2000, he missed a medal by 0.15 sec when finishing fourth in the 200 metres breaststroke, an event won by Domenico Fioravanti of Italy. He joined with Matt Welsh, Geoff Huegill and Michael Klim to claim the silver medal in the medley relay behind the United States. A year later, at the 2001 Fukuoka World Swimming Championships Harrison teamed with Welsh, Huegill and Ian Thorpe to win the gold medal in the medley relay. His international career finished after Athens 2004, where he swam in the heats of the 200 metres breaststroke.