LEGO Australia have made a deliberate decision to stop importing the kits. A new version is due out in the Northern Hemisphere’s Autumn and so they are running down stocks. Given the new version wont launch in Australia until September at the earliest that makes for an inconvenient gap.
The official LEGO store is no longer selling the kits to Australia; although they remain available in the USA if you have any way of buying there. There are also stores in the USA, Toys’R’Us for example, that seem to still be selling Mindstorms NXT 2.0 and some will ship internationally.
Finally, of course, there’s Ebay.