Bit late but just to report back for future reference, we arrived on time and were through immigration very quickly. The agent at the transfer desk said we could bring our flight forward but only apparently by about half an hour, bau also suggested we try doing it at the domestic terminal.

We walked to the domestic terminal in about 10 minutes and used the self-service check-in machine, which automatically offered the chance change the flight - there were many more options offered than the single flight the agent had mentioned, and we got on a 0715 departure - so 2 hours earlier into Queenstown than planned.