If you want to do something you wont be able to do anywhere else, Gisborne (east coast of the north island) is the 'first to see the sun' on new years, basically the first city to experience new years, because of the time zones and whatnot. (I think theres a small island which has the official first new years, but Gisborne is more accessible.) Gisborne hosts Rhythm and Vines, a great New Years concert, held in a really nice vineyard, and with some pretty good acts, relaxed lazing around listening to music and sampling some of the NZ wines. This year the festival is 3 days, whereas before its only been one, so should be bigger and better. Theres a rhythm and Vines campsite that opens, so you will be surrounded by like-minded people, and sure to meet a crwod to hang out with.
Just an option.
As someone earlier said, the trip over the glaciers is pretty much your onl chance of snow at xmas, good luck with that!