I agree with sparky, I personally prefer the .com.au.
If it's for the australian market I will use the .com.au and then try and grab the .com, .net (maybe) and .net.au and redirect to the .com.au
I have quite a few geo domains in all extensions. My .com sites are earning more than my .com.au sites, but that would have a bit to do with my .com sites being older and more developed. In fact, my top two earning geo sites are both .net!
I don't think it matters too much to be honest. Maybe for the Queensland town of Texas you would be better off with the .com.au but for most cities and towns that have relatively unique names I don't think it's an issue. As Jam said, just try and get both!