Casson had a solid second season for Western Australia, taking 17 wickets at 34.11 in the Pura Cup and 4 wickets at 33.50 in the ING Cup, however he missed part of the season due to strained ligaments in his bowling hand. After recovering from his hand injury, Casson had mixed fortunes in the 2004-05 Australian season. He struggled with the ball and bat in the 2004-05 Pura Cup, taking 9 wickets at an average of 40.33 and scoring 36 runs at an average of 9.00. In contrast, he performed well with the ball in the ING Cup taking 8 wickets at 17.75. The 2005–06 season was Casson's last for Western Australia, and he performed poorly, managing 17 wickets at the hefty average of 54.29 in the Pura Cup and only 3 wickets at 41.33 in the ING Cup.