What I'd try and do is use 90K for Business on UA. That's definitely the best value of all. Oz flights are 747 and UA has good seats in C on these flights, esp. in the Upper Deck.
If you're definitely going to be in coach, I wouldn't spend the extra 20K to fly on QF vs. UA. UA coach is ok, and if you have elite status on US or UA, you're entitled to sit in Economy Plus, which makes a HUGE difference. Save the miles in either case.