Assuming you bought the Aussie map from TomTom........
Use Home, Manage my Unit, Computer tab, maps, see if the Aussie map is showing (it should). If so, click on Install.
If Home attempts to do so and says there isn't enough room. I would use Explorer to look at the contents of the unit and copy your UK map folder (and contents) to the computer. Once you are SURE it is on the computer then delete the folder from the unit.
Now Home should be able to install the Australia map.
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