If you have HKICPA, I reckon you can just switch to CA and CPAA.. You do not have to sit for the exam again. CA may have a transitional program, not sure, you can call and ask... If you would like to work in Tax and audit, get a CA is much better...Otherwise, get CPAA would be enough...
But bear in mind, aus coy values local experience more than qualification..... so get in any local firm first and then look for a better job later is not a bad idea