I've has similar connections through LAX in the past - our TA will normally quote a 2hr connection time there - and have not had a problem - even with arriving into LAX T4 on a QF international flight - waiting what seemed an age for imigration to take my photo and fingerprints, then shuttle bus around to T8 (I think - maybe it was T7) to clear check in and security there. Like you, I was nervous about the times, but it all worked out in the end - I waited about 30 mins after clearing security to board the flight. I think that as you will be arriving on a domestic flight (no queues for imigration outbound - only the security checkpoint), then transferring to T4 you will be fine unless your inbound flight is delayed by more than an hour.
Even if you are delayed, QF have other options to get you to Oz - QF108 - 80 mins later than QF12 as well as other options via MEL (QF26,QF94) and (depending on the day) - BNE (QF176) and SYD (QF150).
PS. I am making the assumption that you will be able to check your luggage from LAS all the way to SYD on US Airways - I've done it in the past with America West (now part of US Airways) in the past sucessfully.