Im in South Australia, 1500km from Western Australia. What I know of the W.A. laws is 2nd hand from a friend who is a gun smith, so Im not sure of the 303.
Pump action shotguns are illegal Aust wide, but Pump action riffles are ok, subject to the 5 round limit.
The Government is gearing up for the next round of firearm cut backs, and there is talk about semi auto pistols on the hit list next. But this is only seculation talk from those in the industry because the Gov. hasnt said anything yet.