Personally, I would not go out of T3 QF dom secure zone, and just use the QF transfer bus.
If you go out into the open zone, you are practically "fighting" with VA and the other airline's pax to use the free service.
Even with HLO, and same PNR all the way to the US, or on 2 separate PNR (PER - SYD), and then SYD - DFW.
(**IF**) you decide to get to SYD the night before, then maybe getting out into the open zone at T3 and getting the free shuttle (not QF), might be required, but if you get the red eye, stick in the T3 dom secure zone, ask the girl/guy at gate 15 if you can use the QF transfer bus though, they can say yes, or no, either the night before or flying the red eye.