South Island recommendations
Racers Edge in Wanaka has nice rental bikes, and you definitely dont want to miss the Tuesday evening group ride that meets at the shop.
Vertigo sports in Queenstown is pretty involved in the local trailbuilding scene, I'd definitely chat with those guys. Unfortunately I had endless problems with their rental bikes but they kept going out of their way to make everything right.
Queenstown is badass, I'm ready to give up winter to spend a summer down there.