Coming from Brisbane, Queensland, and of Maltese origin, Callus, graduated in Iona College, he was coached by Chris Urquhart, spent the beginning of his career as the understudy of fellow Australians Michael Klim and Chris Fydler. After finishing fourth in the 100 m freestyle at the 2000 Australian Championships, Callus was selected to make his debut at the age of 21 at the 2000 Sydney Olympics in the 4 × 100 m freestyle relay. Swimming with Klim, Fydler and Ian Thorpe, the Australians were not expected to win, as the United States had never lost the event at Olympic level. However, cheered on by a raucous home crowd, Klim set a world record 48.18 s in the leadoff leg, before Fydler maintained the lead, handing Callus a half-body length lead over Jason Lezak. Callus did well to stave off the faster and more experienced Lezak, giving Thorpe a slender lead. The fast-finishing Thorpe managed to overhaul the fast-starting Gary Hall Jr. by a handspan, sealing a gold medal in a world record time of 3 min 13.67 s, almost two seconds below the previous world mark.
At the 2001 World Championships in Fukuoka, Japan, he was again part of the 4 × 100 m freestyle relay team which won gold ahead of the United States, and was also a finalist in the 100 m freestyle. The following year, 2002 saw his finest moment as he won individual gold at the 2002 World Short Course Championships in Moscow, Russia in the 100 m freestyle.
In 2004, Callus qualified to represent Australia for both the 50 m and 100 m freestyle, as well as the relays at the National Trials, swimming to his usual standards. However, at the Athens Olympics, Callus swum much slower in the heats of his individual events and was a long margin from even qualifying for the semifinals. He was ignominiously dropped from the final of the 4 × 100 m freestyle relay, where Australia came sixth. It was revealed that he had contracted the Epstein-Barr virus and he took a year off to recover, returning in late 2005, declaring that he had completely overcome his illness. He has gained selection for the 2006 Commonwealth Games in Melbourne.
At the 2006 Commonwealth Games, Callus anchored the 4 × 100 m freestyle relay team to silver, and also reached the final of both the 50 m and 100 m freestyle, although he was considerably outside his best time in both events. He took a new gold for Australia in the 2006 World Short Course Championships at 4 × 100 m medley relay, in Shanghai, China, April 2006.
At the 2008 Summer Olympics, Callus was part of the australian 4x100m freestyle relay team, which won the bronze medal in the event. He also took part in the 50m freestyle finishing a close 4th. Callus announced his retirement shortly after the games.