Following Super League, Girdler's standing as one of the best centres in rugby league was cemented when he was then selected for New South Wales in State of Origin. As the Super League War had ended at the end of 1997 with the Super and Australian Rugby Leagues merging to form the National Rugby League, the 1998 State of Origin series saw players from both the SL and ARL available for Origin selection (prior to 1998, Origin selection had only been open to ARL aligned players from 1995–1997, though SL had their own 'Origin' in the form of the Tri-series which also included New Zealand).