There are a couple of methods that can be used:
Method 1 - direct gmapsupp
Download the latest gmapsupp.zip file from http://www.nzopengps.org/public/Direct_gmapsupp/ onto your computer
Unzip and extract - it contains a file called gmapsupp.img - save this to a handy location (desktop is fine)
Connect your GPS with a USB cable - once the driver have installed etc you should be able to use Windows Explorer to see the device.
Variation 1 - copy the gmapsupp.img file to a folder named /Garmin on the Nuvi - this is typically older Nuvi devices
Variation 2 - create a new directory called /Map on the Nuvi if it doesn't exist, and copy the gmapsupp.img into here - this is typically where it goes for newer Nuvis
Jump to step 7 below in the loading from Mapsource section
Method 2 - Loading from Mapsource:
Step 1 Download the latest ********_FREE_OpenGPS_NZ_Maps_Setup.exe from here. The ******** at the front is the date in year month day format.
Step 2 Install the maps, click .exe file. Maps will be installed to C:\Program Files\NZ Open Autorouting GPS Project.
Step 3 Open Mapsource and select Free Open GPS NZ Autorouting map. from the product selection dropdown box. The product selection box is in the 'View' toolbar, you may need to turn it on to see it. The menu path is View| Show toolsbars | view
Once the product selection box is on the MapSource toolbar, you should be able to select the 'Free Open GPS NZ Autorouting' map from the drop down list.
If you dont have Mapsource Read this.
Step 4 Using the Yellow Polygon Map tool, Select all New Zealand map tiles (9 total, 8 regional = 1 POI map).
Step 5 Click send to device, then find device. Choose your Nuvi or SD card from the drop down list. Click Send.
Step 6 Maps will be installed in the Garmin folder (gmapsupp.img) in either your Nuvi or SD card depending on your choice above.
Step 7 On your Nuvi make sure maps are selected. Tools, Settings, Map, Map Info. There should be 2 listing of Free Open GPS NZ Autorouting, the POI map and the Street map. Make sure these are both selected.
Step 8 Maps will only be visible if you have a satellite visible and are in New Zealand. If you are not in New Zealand zoom out and drag map to New Zealand and zoom in.
Note - when doing an address search on a Nuvi / Zumo, use the search all cities option rather than the spell city option. Our streets are linked to suburbs which can be pretty hard to search. You may need to also input the search in CAPS (uppercase). If you have to use uppercase, you may be able to source a special keyboard file for your Nuvi - check out http://gwprojects.org/forum/viewforum.php?f=20 for further instructions.