Remember if you decide to go for a pass (QF only) you must buy it *before* you enter Australia, and also note I think they restrict the sales of said passes to a tie in with an international tix into Oz - ie: I think you have to fly QF (or possibly a OW partner?) into Oz to tack on the airpass.. that may not be the case though - call your local QF res office to find out the exact cost and for any gotchas like that).

DJ (Virgin Blue) only has a tenious codeshare agreement with UA, but it's not very relevant to this situation. Best bet is, as the others have said, is to trawl qantas.com.au, virginblue.com.au & jetstar.com to find low fares on the sectors you're interested in.