B.Com/B.Arts....
Brad, in all honesty if your raw UAI (excl. bonus points etc) is ~85ish, then you should be looking at your priorities and where you would be willing to alter your plans a little.
Forget the double degree for the moment if you're intent on USYD/UNSW as your GPA plus UAI wont be enough to get you into the 98~ B.Com/B.Arts double degrees.
In my opinion, you have two realistic options here:
1. Look elsewhere if you're intent on doing the double degree (UTS/UWS/UOW/UNCLE etc); or
2. Focus on applying for the B.Com or B.Arts single degree, and then if you do well enough once in UNSW/USYD, then look at transferring across to the double degree.
Look at other options.