I bought the CX package (air ticket + Car rental + Motel) and went to NZ in December (more than 5 years ago). I spent around 16 days and the journey to cover both South and North Islands was wonderful. You can see the lakes, snow mountains and even glacier in South Island and enjoy the sunshine and ninety mile beach in North Island. Milford Sound was also wonderful.
If you like cooking, you can buy food in the supermarket and cook your own food (e.g. Steak or lamp chop) in the Motel. Unlike Australia, kitchen facilities are available in the motels in NZ.
I also would like to go one more time in the next few years.