I'm unsure on what you exactly did but here is the way it should have been done...
When you put any SD card in a ONE model, your tt system ignores your internal memory. That implies you need to copy everything from your tt internal memory to your 2GB SD card. This can be accomplished by doing a backup of your system to your hard drive using Windows explorer. If you have a card adapter for your SD card, use it and copy your hard drive backup onto your 2GB SD card. If you don't have an adapter for your SD card, you will need to put it in your tt system and link it to your computer with your USB cable. You should see your 2GB SD card and copy all your internal backup data to your 2GB SD card.
Disconnect your tt system from your computer after making sure everyhting has finished copying. Turn off your tt system and turn it back on. If everything was copied correctly, you should have a working tt system from the SD card instead of internal memory.
Once you have a working system with the SD card, you can start to think about your Australian map. Your map would have been downloaded on your hard drive in the Tomtom directory, and the map subdirectory. It is now time to add it.
Once added, you have to select which map to use in the preferences. Navigate thru it and you should see ?change map? and you will be able to choose between your two maps.
I don't know the size