hi I'm planning a trip to NZ for March too, I'm going to do one of those working holidays, so I'm starting out in Hastings, Hawkes Bay because there is a lot of fruit picking work which pays ok so gonna use that money to fund my travelling.
There is a website ive came across called http://www.bbh.co.nz/ which lists a lot of info of accomodation, although I think its just hostels but I think its pretty good, you can send away for a free brochure to get some more info.
I'm also thinking about doing a magic bus tour of the whole of NZ to make sure I dont miss anything because pretty much want to see it all lol, the website is http://www.magicbus.co.nz/
The tour guides are supposed to be really helpful and they go stop at all the good places, you can get on and off as often as you want, i.e. get off the bus and spend a few nights in a hostel so that you can go skydiving or something crazy, then get on the next bus and go somewhere else and do something else like bungy jumping lol. To cram the full tour in really quick it will take a minimum of 26 days to do.
Another cool website is this one
(its one big link by the way so copy and past both lines together)
http://www.newzealand.com/travel/about-nz/features/go-all-the-way/?cid=uk0909_search_paid_google&gclid=CJOI2duAzp0CFVtn4wodGS_rrg
There are the i-site websites aswell
http://www.newzealand.com/travel/i-sites/i-sites_home.cfm
Erm.. I cant think of much else that will apply to you because all my info is for working holiday stuff lol.
Oh there are companies that are supposed to be really good for hiring cars if you are wanting to do a road trip, i want to do that aswell but I'm gonna be over there for at least 6months - a year or more because on a working holiday I can stay for up to 2 years, hiring a car might be a good idea for you all to travel together, or even a camper van lol
Good luck with your trip
:-)