I have a few geo domains, and a couple of my .net sites rank better than their .com.au equivelant. And in some cases, I'd say my .com.au competitors are better sites than mine!
On the subject of registering the .com to match your .com.au, a few years back a mate of mine registered a .com which matched .com.au of a relatively well known Australian company. His standard policy was to set up catch-all email accounts for each of his domains (a recipe for spam if you ask me!). Anyway, he received HEAPS of emails directed at the real company, where the sender had just left of the .au from the email address. He also received plenty of type-in traffic.
So yeah, I'd say registering the .com (if you can get it) is massively important if setting up a .com.au site!