Sleeping Bag Rating

I am not sure what your intentions are but I hope you have researched the Te Araroa Trail. It is a bit of a "virtual trail" really. It is more of an idea rather than something that is developed on the ground and is not promoted to any extent. A lot of the route is in front country areas and will inevitably be walking along roads. More hiking (tramping) in new Zealand is done on backcountry routes which have never quite connected into a country length trail like the Te Araroa proposes to do. If the Te Araroa is what you intend to do then you won't need much of a sleeping bag in Jan-Mar. It is high summer and most of the trail is at relatively low altitude. However, in saying that New Zealand's climate can change rapidly and you could get a good southerly blast even in mid summer to cool things down. Then if you go into the back country and get a bit more altitude it will be cooler. My philosophy is that I take the same sleeping bag all year round but it has a full length zip and a lot of the time I sleep with the zip open.

There are a lot of ifs, buts, and maybe's when trying to answer a question like this, but I'll try. If you start from Cape Reinga heading south, it is highly possible to encounter snow in the southern half of the SI during March. So the 25F would be nice then. Up until March though the 45F bag would be a better choice. (typical day temps low 20's to low 30's degC, pm temps probably drop to 7-8 degC) If you start from Bluff heading north then the 45F probably would be sufficient.