You can't register .au - only .com.au, .net.au, .org.au, etc. Most .com.au registrars are pricey - up to $99/year, although there are a few places selling them for as little as $49.95 for 2 years. I have used Destra and have not had any issues with them.
There are strict requirements for .com.au registrations and I think also for the other extensions. The name you register must relate directly to your registered business or company name for example, so you can't easily go registering tonnes of names.