the weather usually gets cool at night in the mountains even in summer , the temp will often be somewhere in the teens at night. although a full hut will add a few degrees to that.
i usually find the exped ultralite 300 more than does the job. will cover you for most days at that time of year including some of the cooler weather as long as you're not sleeping out in the alpine zone in bad weather.
http://www.exped.com/new-zealand/en/pro ... 300-m-left you can move all the down to the top of the bag for cooler weather to get more warmth out of it.. half a kilo, packs down super small with 850 down... BUT its a narrow bag, when its zipped up you can't get your arms out of it because it even tapers in around the neck before you even pull the drawchord, if you feel claustrophobic in narrow bags, dont buy it,, and you have to be careful with the zip . no bells and whistles, no neck muff, zip is only two thirds the length of the bag
if you want something roomier but still pretty light, I can also recommend the Exped Lite 500 which i had before exped released the ultralite range and i got the 300,. Although it will be generally too warm unless the weather is bad. its a good general purpose three season bag I find if you dont have the budget for multiple bags.
http://www.exped.com/new-zealand/en/pro ... 500-m-left
from the land of the long white clouds...