I had a Garmin Oregon 400, and bought with it Garmin's own topographic maps called "TOPO Australia" for an extra $210 from RYDA. It was RRP $235.
The oregon was sold, now i am selling the the maps. it is in microSD format and is very easy to use.
from their website:
Use with Garmin’s Colorado™ or Oregon™ series navigators to unleash the full feature set of TOPO Australia, including full 3-D view and terrain shading. Also compatible with the following units: eTrex Legend® Cx and HCx series, eTrex Vista® Cx and HCx series, GPSMAP® 60Cx series, GPSMAP 76Cx series, Edge® 705, nüvi® 500 series and zumo®.
I bought it mid last year and selling now for $50 delivered Australia wide.