We lived in the south island; all I can say is brrrrrrrrrrr.
Our house which we paid $300 a week for (which was cheap mind you in the area we lived in) had no insulation and the heatpump sucked big time; landlord was too cheap to insulate and fix the heatpump which shutdown when it was too cold outside.
The majority of houses in NZ do not have central air so you will be pretty much stuck in the living room (or where ever the heatpump/fire is located in order to keep from freezing in the winter.
We basically lived in the livingroom/kitchen in the winter during the day and at night braved our bedroom which was so cold we would wake up with moisture on our bed from the cold damp air; moisture would also drip off the ceiling and on to our faces during the night.
$300-400 power bills to freeze at night and to live in one room.
This occurred in all 5 of the houses we lived in while in NZ.
The south island is better imo for the outdoors people BUT then again you have to make enough money to afford the price of gas in NZ to be able to travel to get to the areas of recreation; and get a day where it is not raining.
Nelson was nice when we were there but found it to be a town where with EXTREMELY small town politics, gossip and pretty much a good old boys network which makes getting work (when there is work) almost nil.