Do I need a tent

I presume you are coming from outside NZ so you will find that NZ has a comprehensive network of huts maintained by the Department of Conservation (DOC). Many of them are on this site but a more comprehensive list can be found on the Department's web site http://www.doc.govt.nz/templates/ByRegionLanding.aspx?id=37324 The answer to your question is, it depends where you want to go. Many of the most popular routes have good huts on them but if you want to get away from it then you may need a tent in some areas. Huts in NZ tend to be quite basic. There will be bunks with mattresses, a water supply, a log burner and long drop toilet. Most have no cooking facilities though some of the more popular ones do have gas cookers. You will need to carry all your food with you. Unlike some places, no huts sell food and you will have to carry all your rubbish out with you too.

It can be handy to camp before a tramp even if you don't take the tent on the walk -- e.g. drive to Arthur's Pass Friday night, camp, set off first thing. There are generally lots of places you can camp in wild areas. You could buy a relatively basic tent for this purpose. Note that it will need to be very clean when you take it through the airport. :-)