Have replied on the OP forum regarding the simple street map install. Not the MapSource / BaseCamp method yet.
To add the Open Project topo to your GPS, download the ready made NZ Open Project Topo v5.2+.img from my Dropbox here
It's a slow but ad-free connection. Put the file in the Oregon > Garmin folder.
Note, it has no roads as it is made to work with the OP street/POI map.