I'm doing the Queen Charlotte Walkway in four weeks time, so I'll be able to give you a first-hand account. Bear in mind that about half of that is closed to MTB traffic Dec-Feb. There are heaps and heaps and heaps of places to ride in the South Island. If you manage to be here in the school hols I'll even show you some of it. To get the most of the best riding, renting a van or something would be the way to go. You could still do plenty of 2/3 day rides, but you'd miss the best of the singletrack trying to lug paniers or trailers.

A sample of the riding that can be done just on Department of Conservation land: clicky-clicky - everything listed from Nelson/Malborough on is SI.