How do you plan on getting from CNS to MEL, and MEL to ZQN (Queenstown)? Qantas only have the 1 service between CNS and MEL per day, which can be quite hit-or-miss regarding loads. Jetstar I believe have more, do you know if you have an agreement which covers them? Also, the Qantas MEL to ZQN flight is seasonal, and not operating at the moment due to summer. Air NZ however do operate this route.

Also, have you considered flying Queenstown to Auckland, then back to LAX? By traveling to SYD you are back-tracking the Pacific a little, and Qantas has recently changed Auckland to Los Angeles back to a 747-400, and less likely to suffer a weight restriction ex Auckland than Sydney.

Whatever you decide to do, have fun! And enjoy the aussie sun.