Australia to NZ is a three hour backhaul from SYD, although it then enables you to fly to the same jet ports as AKL connections but clear customs at a smaller NZ airport and without the inter-terminal bus ride or 15 minute walk (especially if you want a regional connection.
As Mwenenzi says, accommodation would be worth nailing down first for that time of the year.
To get the best of NZ do an open jaw trip - arrive in one port and depart from another and road trip the ground time. As I am biased I would recommend arriving in to CHC and departing from ZQN and take a slow drive between the two as there is plenty to see and do
UA from SFO to CHC may be a helpful option