The military option is the most sensible way to go to become a test pilot for those in Australia. I did it that way and now work in civil industry in the US.
However there are other Australian military options to the RAAF. Both the Army and RAN have test pilots. You will need a minimum of 1000 hours pilot in command of operational flying prior to selection for TP course. All Aussie military TP's are trained overseas at one of the TP Schools - either Empire (UK), USAF (Edwards AFB), USNTPS (Patuxent River, MD) or NTPS (Mojave).
If you have money you could pay your own way through NTPS - it is a civilian school open to anyone (
www.ntps.edu) and awards a Masters degree on completion.
You are headed the right way. An engineering degree is desireable but not mandatory. A BSc is fine, particularly if it is in maths or physics.
Good luck. You have a long path to travel.