How cold are the huts on the Milford track

Greetings all. Heading to the Milford track in January. Previous walking in s w tasmania and ski touring nsw so we are set up for quite cold temperature. I am wondering if we can get away with lighter sleeping bags as I have heard the huts on the Milford track are pretty luxurious and full of warm bodies and stoves. Would a bag that was good down to say +5 or even warmer be adequate? I am usually the family packhorse but my knees are not what they were, so weight is becoming more of an Issue. And if anyone has any recommendations for a nice light bag that suits let me know. Thanks in advance Adrian.