Discovered in 1993 by GoldRats Rugby Union Club of Southern Cross University, Lismore, New South Wales while studying coastal and fisheries management,[citation needed] he then moved onto the Tuggeranong Vikings Rugby Union Club in Canberra, he made the 1994 Australian Capital Territory (ACT) Under-21s and then the Australian Universities team.

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[1] from which he graduated in 2001.

[2] 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.

Harrison proved an effective player in the Celtic League, a consistent winner of line-out possession and aggressive in rucks and mauls.

On Monday 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.