The trails in Whakarewarewa (Rotorua) and Makara Peak (Wellington) are both good options for XC singletrack. Both are easy to reach from town (rideable from the centre) and can be plenty of fun without needing a guide or getting more epic than expected. Both have trails from easy through technical to downhill, clearly marked and graded. Rotorua hosted the 2010 SSWC, but Wellington has better beer
Mud Cycles in Wellington is about 200m from the trailhead (they just moved to a new shop nearer the trails) and hires basic but usable hardtails.
There are loads of utterly stunning epic rides (the Heaphy track is now open part of the year) but one of the trail centres is a better idea if you don't have a guide, lots of your own equipment etc.