Here are some tips provided by Frank Zgoznik for using the Garmin Vista HCx
Map Limits on the GPS
The Garmin units have a limit of 2024 maps. So not all the OZ topo maps will fit on the GPS.
I found that I could install all the City Navigator maps (MetroGuide is your equivalent), all the Tracks4Australia 1.20 maps but only the eastern half of Australia using the OZTopoMaps10m.
Technical details here: http://freegeographytools.com/2007/garmin-microsd-card-tricks