Road conditions are excellent in NZ (even secondary roads are well maintained). However, the more isolated parts are surprisingly isolated. Even in good weather I've been on main roads and counted 1 car per hour passing by. Not at all like the more populated parts of the world. And in some parts, such as the west coast, motels and garages can be quite far (so you need to plan your route). As for the weather, the ferocity of storms is quite impressive. The average NZ west coast storm is close to hurricane intensity (at least for rainfall and visibility while driving), something that most people are not expecting. Sure there are only a few storms a year when roads are closed, but if it happens on your trip and schedule it can become quite a problem. Also there are few alternative routes. Overall the weather in NZ is wonderful, but if on a driving tour with an airline flight at the end that you cannot afford to miss, you have to keep the chance of delays for a day or 3 in mind. And the risk isn't academic, several tourists have died during winter in the south island in the past 2 years (and not just from mountain climbing, but by driving off the road). I'm tuned in to this as during one trip in NZ I saw a car spin out and go down a gorge, and this was in fine weather on the north island. Driving in rural NZ requires a bit more skill and attention than some other countries, but is essential to seeing the real NZ.