The Qantas Academy is no guarantee of a job with mainline. If you do get through you may be placed on the hold file for the Qantas group. Most likely that would lead to an offer in Qlink or Jetstar (possibly Network aviation, Jetconnect, EFA). If you end up with one of these the door is not shut to mainline but doesn’t make it any easier to get in. It would be extremely doubtful that at completion of the course you would get any kind of direct entry as an SO into Qantas.