Accommodation is most often provided courtesy of DOC in fabulous weatherproof huts. Many of the recent huts are insulated and even double glazed. In more remote valleys the original huts have neither, often there are louvre windows and plenty of ventilation via the open fireplace chimney, although almost all huts have plastic encased mattresses. Combine that with 41º—47º S latitude, the elevation and deep, shady valleys and, well, it gets cold at night.
Sometimes real cold.
The South Island is classified as Cool Temperate: moderate days and cool nights in summer, cool days and cold nights in winter. Not often deep snow, blizzard cold, but substantial, frosty cold.
A -6º C bag, (21º F), should be adequate for most nights. If you are at altitude, or hit a cold snap, bung on a few extra woollen layers, and throw your jacket over the top.
Let’s get contentious: feathers or synthetic?
Feathers are compact and warm. Problems are cleaning and what happens if you get you sleeping bag wet. To generalise and oversimplify, feathers are better in the cooler areas.
Synthetics are more bulky for the same warmth but are easy to wash and just as warm when wet. OK for summer.
In any case get a decent silk liner sheet. Much easier to wash that regularly than the bag itself.
Remember it’s a bitch to be cold night after night.