Tony Savage (Kiwitonita) http://forums.gps.org.nz/memberlist.php?mode=viewprofile&u=24
is more or less our default GPS supplier even though he's in Christchurch (just an overnight courier away). He's an official Garmin retailer so you're not getting parallel imports, his service is exemplary, and his prices are at the good end of the scale. You can get your unit as it left the factory if you are comfortable setting it up yourself, or for a few extra dollars Tony will take care of all that so when it arrives you turn it on and it just works.
There are any number of places in Auckland where you can look at the Garmins and compare displays and see how they feel in the hand, but when you're ready to hand over the dosh Christchurch is the place to go
(unless you find someone accidentally advertising them for -$50