It is a lovely time to come and visit as there is heaps on to do and see. However, down side is that accommodation books out fast, especially for the duration of the Sydney to Hobart yacht race.
As for weather, well one cannot always tell. I remember a few Christmases with snow on Mt Wellington
, lots with rain and others that were scorchers. Very hard to predict. However, just remember that a 25 degree day here is a lot "hotter" so to speak then a 25 degree day in other places. I have lived (amongst other places) in Sydney and Melbourne and the heat is a lot more intense down here.
Happy Planning, I'd get on to the accommodation ASAP.