As others have noted, it's possible to build two versions and to use the iTunes store to restrict distribution based on the nation where the buyer is located.
However there's another issue to consider. You say that the problem is "accessing the content via the app" outside of Australia. But you're on a platform which is inherently mobile, and may not be located in the same country it was when the user got your app. What happens if someone is in Australia, gets the app legitimately, and then travels to some other country? They're no longer in Australia but they have a copy of the app that assumes they are. Would your client's copyright restrictions still permit them to access content that's supposed to be Australia-only?
If not, your problem is rather different than using app-store restrictions. But it might also mean that you get to have a better solution to the problem that doesn't require multiple versions of the app.
Think about it: the iPhone knows where it is. A 3g phone can get GPS-accurate location info, and the older phones get reasonable approximations anytime they have a phone network connection. That gives you lat/long coordinates.
What then? Since your home area is pretty well defined, it might be simple-- draw a rectangle around Australia on a map, and define "in Australia" as being in that rectangle (being a whole continent comes in handy here). If you want something more precise, feed that lat/long into a reverse-geocoding API (there are many-- Flickr has one, for example) to find out what country you're in.
And voila, you can make the app behave one way in Australia and another way outside of it, and you only need one version of the app to do it.