I'm almost nearly, very sure, that the basemap doesn't display at zoom level 'x' or 'y' as such!
It's more like..... each layer (0-10 in the topo maps) does or doesn't display at zoom level 'x' or 'y'. It's built into the map compilation, and described in a file called map00.ini
Therefore the basemap detail takes over. at zoom 'a' or 'b' ..but not completely!
It would be nigh on impossible for the basemap to display topo contour lines at zoom level 5km, let alone 350 meters, so that info comes from the topo SD card
Yet at 5km I lose the Calder Freeway at about Woodend/Macedon, and the basemap only shows the "old" Calder hwy from about there.... so that's what I see. At zoom 2.5km I see the Calder Fwy again.
So it's not a matter of "the basemap takes over at zoom X!!
Also, the Aussie basemap is quite good(for a basemap)with lots of main roads detail.
Hmmm....there's about 8 megleft over to play with there!!!
Using the US basemap, the Aussie mainland may be based on WorldWideBaseMap, which can only be described as a joke...check the water around Port Philip Bay, and prepare for some scuba diving whilst driving around the Brighton Beach area
Like everyone else said...get DAST v1 or Topo...forget about the basemap! Keep the zoom level within about 10-20km