If you don't have to stop in AKL, United Airline has SFO - SYD - MEL. Depending when is your trip, the price for late Nov / early Dec is US$1300 (including tax and fees).
Another option maybe flying non-stop UA SFO - SYD ($1140 for above time frame), and buy separate ticket for SYD - MEL from Qantas or Virgin Blue.