In 2004, he moved to the Brisbane Capitals. Then from 2005 to 2008, he played for the Southern Districts Spartans (now known as the Brisbane Spartans).
Williamson's first senior season came in 1998 playing for the Maroochydore Clippers of the QABL, and continued to play for them until 2003, winning club MVP from 1999 to 2003. With the Clippers, he won the inaugural Southern Cross Division title in 1999, was the ABA's steals leader with 4.13 per game in 2003, and earned co-MVP and Co-Youth Player of the year honours in 2003.