He moved to the Tuggeranong Vikings Rugby Union Club in
Canberra, he made the 1994
Australian Capital Territory (ACT) Under-21s and then the Australian Universities team. The following year, he played in the Australian U21s team and made his debut for the ACT against
New South Wales B. He made his
Super 12 debut with the
Brumbies in 1997. He made his international debut as
Australia took on the
British and Irish Lions in the third and final test of the
2001 series; at the time Harrison was a student studying for a
Bachelor of Applied Science in Sports Administration at the
University of Canberra from which he graduated in 2001. A veteran of the
2003 Rugby World Cup, Harrison joined the
New South Wales Waratahs at the beginning of 2004 for two seasons before moving to the northern hemisphere to play for
Ulster. As of 23 May 2006, Harrison had played 25 games for Ulster, 19 of those in the
Celtic League and 6 in the
Heineken Cup. Harrison proved an effective player in the Celtic League, a consistent winner of line-out possession and aggressive in rucks and mauls. Although regarded as inspiring by other Ulster players, his tendency to indulge in
sledging led to confrontation with a number of opposing players and a number of off the pitch incidents. At the end of the 2008 season, Harrison moved to
Bath Rugby. until his retirement after involvement with the cocaine scandal and a bar fight with Harlequins players at Bath at the end of the 2008/09 season. On 30 November 2009, it was announced that Justin Harrison would return to play for the
Brumbies in 2010 on a one-year deal, as coverage for
Peter Kimlin. However, the ARU has not endorsed this contract. ==Post playing career==