With the Garmin you can use map's by Freshmap.co.nz and no doubt other aftermarket providers. You can have them both running at the same time too it would appear, or you can go into "maps" and select between the two. The freshmap also has Tropo maps as soon as you leave the roads. Good for 4x4 ete. If fact the same card will change between a handheld "tramping" garmin like the map60 series and the nuvi series for street finding.
Having just got a tomtom for Christmas, i would certainly recommend it if you are still shopping around for one. The older map version of NZ did have missing streets but they certainly have been rectified in the latest version of the NZ maps, which incidentally are NOT part of the device but as a few have already mentioned, you can download it for free within 30 days of your product activation.
The biggest key advantage for me with the tomtom is that it has both the device and PDA version of the application. The PDA version (Navigator) does not yet have NZ maps but when it does come out, it will look exactly the same as the tomtom one XL device. POIs then become interchangeable and things like favourites can be brought across without any problems. Voices that i purchased through tomtom can also be transferred across with no problems.
The device itself has been great. Easy to use and very fast recalculations of alternate routes, especially when you have missed your turn. Downloading updated maps is real easy now with the latest PC version of the tomtom home software which comes coupled with the device. The website is the other alternative to buying maps so you dont necessary have to use the HOME software.
I love it and would certainly highly recommend it.