Lots of different options depending what you want.
The classic areas not to miss for the best classic mountain biking are
Nth Island: Rotorua, Taupo, Wellington
Sth Island: (Too many to list!!): Nelson, Christchurch (including cragieburn etc), Wanaka, Queenstown
But there are LOADS of lesser known spots with great riding. If you want to any biking in NZ just get your hands on this:
Classic New Zealand Mountain Bike Rides – North and South
Most Kiwi bikers own a copy!