I am afraid at this point in time the IR has to be done in the UK.
At the moment, the Aussie $ is quite strong against the Sterling so it's not too painful.
Where you do your ATPL theory is up to you. If you don't use the Western Australian operator, then I highly recommend Bristol Ground School.
To summarise. Use the CAA UK LASORS for reference.
To convert an Australian ATPL to a JAA one. (ref the UK CAA)
UK CAA Class 1 medical, Can only be done at Gatwick. You'll get a discount as you already hold an ICAO recognised ATPL.
14 ATPL exams. They are so much fun. You'll love them.
The flying side is tricky and depends on your hours etc.
If you have no multi crew/multi pilot time.
You'll need to do some flying for the CPL as well as the IR.
You'll get a CPL.
If you have 500 hours of multi crew but in a Bandit or something like that.
You could do the above or get a type rating on a B737 with the LST observed by a CAA FOI.
Depending on what you do, you'll get a CPL or an ATPL.
If you have 500 hours on a Dash 8 or B737 etc, you just need to do an observed skills test on that type. Note, I recommend you get some flying in the sim prior to the test to learn about the unique UK rules....
You'll get an ATPL.
Note, if you hold a CPL, most operators will want a MCC (multi crew certificate) before hiring. Yes, yet another expense.