I realize you're referring specifically to general .au, but I don't think the average person will think to try foo.au instead of foo.com.au and .com.au isn't opened up to my knowledge. The name must be derived from your registered Australian business (ACN or ABN).
I've had a .net.au since 1999 and the same rules apply there.
The domain does not have to be derived form anything, only associated (owned?) with an ABN/ACN. And the bar for registering an ABN is as low as signing up for an email account.
People in Australia use .com.au all the time, domain.au almost looks broken to my eye.
Investor value at the cost of hurting small business by increasing cost or creating confusion.