Something to be aware of for the very peak travel days around Christmas / New Year is that often flights may be upgauged to larger aircraft or even additional flights scheduled if demand warrants. In that scenario, often there are suddenly a few more cheaper seats available again, but it's not something you can count on. Travelling between NZ and Australia at Christmas time is just very poor value, and generally speaking the earlier you book, the better, since they are never going to have fare sales that apply to that most peak of travel periods.