Our main resource for planning our trip was http://www.purenz.com/. This website suggests various itineraries depending on the length of your vacation. It starts out with a high level map, and then you can hyperlink yourself to more and more detail, eventually ending up to a specific attraction or campground.

We also used the following books for details:
Frommer's New Zealand
Moon Handbooks New Zealand

When we were in New Zealand, we picked up free maps by Jasons. These were excellent and readily available. Or try www.jasons.com