You can lay claim to a .com.au if the domain name is "closely and substantially connected" to you as the registrant. This includes things like selling a product or offering a service with that name.

auDA ruled a few years back that "domain monetisation" falls within "a service that the registrant provides" (with a few exceptions).

Sales/transfers of domains were a bit painful back in the day but have been relaxed now. There are restrictions (from memory, you can't register a domain and then sell it within six months) but it does make speculation a bit easier for domainers.