Despite having played most of his senior football in the Brisbane Rugby League competition, Trewhella was ruled to be eligible to play for New South Wales in State of Origin after having first play senior football as a teenager in the Newcastle Rugby League. He was selected on the bench for Game 1 of the 1988 State of Origin series at the Sydney Football Stadium, and at hooker in Game 3 of the 1989 series at Lang Park in Brisbane where he scored his only try in representative football.