You can't tell - and they can change at any time, based on maintenance and other scheduling requirements.

I recently flew LAX-NAN-SYD, all on A330s. I assumed that this would be the same plane, based on timing, but it turned out not to be, as we had a different plane for NAN-SYD.

I would guess that it's unlikely to be on the same plane out of NAN that you flew in on. I think there's other flights to SYD/MEL/BNE that could also use the 737 you flew in on.