Luxlumis, I guess that the New Zealanders are off line at the moment due to the earthquake although there weren't too many of them around in the first place. The South Island is WAY south. On par with the lower reaches of Chile and further south than Tasmania so it is cool down there. Early autumn is the nicest time of year in Canberra which is cool for Australia's capitals so I can easily imagine that Autumn would be nice on the south island too. You still have some summer warmth but no strong winds. Spring is pretty windy in this part of the world so the weather can change quickly.
Late December/January is the major school/summer holiday season in New Zealand and Australia so yes its peak time for accommodation but I cant give you any first hand advise.
Anthony