If you get a chance, stop at a few of the Marlborough wineries in the northern part of the south island...some real gems! Taupo Lake is not to be missed either!!
Oh, and if you must stop in Nelson for the night, there's a hotel advertising rooms for $35 cheaper than anywhere else.... just be warned, the freight train passes within 12 feet of the rear of the hotel @ 3AM.
Have fun!!