Upgrading to the .com is all good and well for big household names, but I wouldn't recommend it for anyone smaller if their target market is Australia.
For Big W and the NAB, everyone knows that they're Australian. But say for my company Trade Risk, I would never run my site on the .com as I need the trust factor that the com.au brings.
Of course I'd love to own the .com just to have it, but I'd never move my site to it. For that reason I'd probably never pay much for it either. It's just not a priority.