Giaan Rooney's first international swimming competition was at the 1998 Kuala Lumpur Commonwealth Games where she won gold medals in the 100 metres backstroke and the medley relay. By Sydney 2000, she was world-rated in the freestyle and backstroke. She joined with Susie O'Neill, Petria Thomas and Kirsten Thomson to finish a close second behind the Jenny Thompson-led Americans in the 4 x 200 metres relay. Rooney's program in Sydney also included the preliminaries of the medley relay, where Australia claimed another silver medal behind the United States, the 200 metres freestyle, the 100 metres backstroke and the 4 x100 metres relay.
At the 2001 Fukuoka World Swimming Championships, she won the 200 metres gold medal. At Athens 2004, she was part of the world record-breaking team, with Jodie Henry, Leisel Jones and Petria Thomas, that won the gold medal in the medley relay, as well as contesting the 100 metres backstroke and the 4 x 200 metres relay. Over her international career Giaan Rooney garnered many medals in individual and relay events at Olympic and Commonwealth Games and World Swimming Championships.