Casey carded an opening 67, five under par, at Lake Karrinyup to lie two shots behind joint leaders Michael Hendry and Alejandro Canizares.
And after two birdies in his first six holes on Friday, the former Ryder Cup star found himself at the top of the leaderboard, with Hendry and Canizares among the later starters.
However, Casey then dropped a shot at the 18th, his ninth hole, and worse was to come with a double-bogey six at the second.
Another six followed on the par-five third and when he also bogeyed the fifth, Casey was on his way to an inward half of 40.
That meant a second round of 75 and a halfway total of two under par, five behind clubhouse leader Bo van Pelt.
Casey has struggled for form this year after suffering a dislocated shoulder in a snowboarding accident last Christmas, although the 35-year-old showed he was on the way back at last week's Shinhan Donghae Open on the Korea Golf Tour with a third-place finish.
Van Pelt added a 67 to his opening 70 to lie seven under par, the American carding an eagle, five birdies and two bogeys.
New Zealander Hendry had just started his second round and opened with birdies on the 10th and 11th to improve to nine under par, one shot ahead of Argentina's Emiliano Grillo, who had picked up two shots in his first five holes on the front nine.