Prime time for NZ boating is November through March (their summer). We got into some great heli access stuff on the West Coast South Island around Easter when we were there. With 5 months you should be able to cram in most of classics if you are game. I would suggest flying to Christchurch, getting a vehicle somehow (we got a sweet ride for $350 at an auto auction), drive to Murchison and do some warming up on the bevy of class III-IV stuff there. When you've made some contacts and gotten your feet wet head to Hokitika and hope for high pressure, or else the heli won't fly. If you want to do heli runs I suggest the Arahura, Whitcombe, Upper Hokitika (with a solid bunch of locals), Perth, Whataroa, Kakapotahi. You should also head down to fjordland and check out the Hollyford. Then there is always the Kawarau (sp?) down near Queenstown, decent play and huge rapids at Nevis Bluff, there are also some good creeks in that area.
The North Island has some great runs, but it will take at least a day of traveling to get to them from the South Island. Up there I recommend the Kaituna, Wairoa, Rangitikei, and Mangahao (if it is releasing). There are also sick surf breaks all over the country, so that is your best option for playboating. Get Grahm Charles' guide book for the real scoop.
"A river is more than an amenity, it is a treasure."
-Oliver Wendell Holmes writing for the majority in New Jersey v. New York