UA flies to Sydney from LAX and SFO.
Usually the jumpseat is available but flights are sometimes weight limited. If you go by flight ops around 9PM the crew can check with load planning.
Out of LAX try standing by for the AKL and MEL flights just in case (they all leave about the same time)and have some extra Qantas/Ansett/Air New Zealand ID tickets for the connection if it becomes necessary.
Good luck - you'll have a great time in Sydney.