The driving in much of NZ is indeed scenic, and/but you need to allow for the fact that a great many of the roads, while designated as main roads, are winding and hilly (mountainous, really). You can have a look at places you're thinking of travelling by looking on Google Maps StreetView, to get an idea. And there are strict speed limits, backed up with speed cameras and hefty fines, even for foreign tourists.
Do you have any possibility of allowing flexibility, so that if you find you are tired out and/or you find you'd like to see more of somewhere, you could stay in place an extra day?