There are a couple of NZ sites you should have a look at that have Garmin style maps. These are:

The New Zealand Open GPS Maps Project for Garmin Mapping GPS at: http://gwprojects.orcon.net.nz/gps/ - Everything on this site is free to download.

Brent's New Zealand GPS Maps: http://nzgpsmaps.com/ - A commercial site but the basic maps are not expensive.

The maps might not be as sophisticated as a Garmin product but they are far better than nothing 🙂

Cheers

Brian Z