He was a right-handed batsman and a right-arm medium-fast bowler who played for Berkshire. Theunissen represented Berkshire in the Minor Counties Championship for three seasons, and made a single List A appearance for the team, in the 2003 C&G Trophy, against Durham. Theunissen continued to represent Berkshire in the Minor Counties Championship until 2004.
In the 2006 Cockspur Cup, he played for North Maidenhead and Falkland.
He also played in the 2007 competition.