if you really want a super cheap flight and u've got flexible timing, check out aircouriers.com (i think).
you can actually fly to some places for free, provided you're willing to travel very light.
couriers to aus. are a bit rare though, so don't hold your breath.
but if you're flying to sydney, expect to pay from 1600 to 2100, economy, with a stop over in LA or perhaps Hawaii.
If you value a good flight experience flying across the pacific i highly recommend Air NZ; they've put every other airline I know to shame in terms of service and overall quality.
good luck!