I'm not sure if you are talking about "free" map updates here, but if you are, then I'm afraid the "Latest map guarantee" and the "Lifetime map update" offers ONLY apply to the map for the territory where you bought the device. So for a "World" model bought in Europe you only get free updates for the Europe maps, and for an Australian unit, you will only get Australia.
It sucks, and TT are not "up-front" about it before you buy (or afterwards either!)
But if you are not expecting free updates, then it's probably just down to the WORDING in Home or the TT website.
There is not really any such thing as a map update, every map you buy is a complete new map. So they just may be offering a price-deal on a new map for the Aust. one you already have, or you may not be realising it is a full-price for it, as it is quite a bit cheaper then the Europe map.