[5] Harrison made his debut in the National Rugby League for the Brisbane Broncos against the New Zealand Warriors on 20 February 2000 (Round 3).

Having won the 2000 NRL Premiership, the Broncos traveled to England to play against 2000's Super League V Champions, St Helens R.F.C.

Shortly after, he was promoted to the Rabbitohs captaincy, which coincided with the team winning six of their final eight matches of the season to avoid the wooden spoon (the only losses in that period being against the eventual Grand Finalists, Wests Tigers and North Queensland).

Harrison was picked ahead of his club captain Scott Prince (who was only drafted into the squad after a knee injury sustained by Darren Lockyer) to play in the second State of Origin on 11 June 2008.

In the 2011 NRL season, Harrison was limited to only 13 games as the Gold Coast finished last on the table and claimed the wooden spoon.