You can edit the KMZ files and remove unwanted tiles (like areas outside the national park, or area you are hunting. I do this for the four maps that cover the Tararua's. I can then have three of the maps displayed on the GPS instead of 1. This works well, and all I need to do is connect the gps (62S in my case) and copy across the maps that cover the area to which I am going to hunt.
I also use the free nz open org gps mapset. So if I go off the custom maps (KMZ) the open org maps are displayed. link below.
NZ Open GPS Project
Seems to work ok but isn't perfect. But to be honest, the better solution will be to have something like freshmap or maptoaster installed. But they cost money and I am a tight bastard!
Maptoaster: New Zealand Topo Maps for your Garmin GPS :: MapToaster Mobile/NZ - NZ Topographical Maps
Freshmap: Garmin™ GPS Topographic Mapping
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