It kinda depends on the map supplier most in car GPS use Naviextra for the mapping, Some units the map is locked to the SD cards serial number and others it's the unit's serial number
The most important files are the license folder files and if by chance you have that folder put that back on the SD card the folder came from and log into naviextras.com and hopefully it will update the missing files.
If the card was supplied as a update there is probably not much that can be done other than maybe a cracked version but getting an australian version map is near impossible.
I think 1st step is to work out what brand of maps it had and check the website on how to update the mapping.
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