I recently needed to buy a new Sat Nav for use in the UK and searched for one that I can use in NZ in due course. I found that open source maps (free, though you can give a donation to the project) for NZ are available from
http://nzopengps.org/ which will work on Garmin devices. After getting my new Garmin Nuvi I downloaded the NZ map onto a miniSD card which the Sat Nav recognises and I can find streets in towns in NZ.
For some reason I can't map a route from North Yorkshire to Auckland ;-) ......