There are no proprietary rights in a domain name. The registrant does not "own" their domain name. Instead, they have a licence to use the domain name for a specified period of time. Registrants can transfer (or "sell") their domain name licence to another person. For more information see the Transfers (Change of Registrant) Policy.
No one has an automatic right to a domain name. Having a company or business name, or a Registered Trademark, does not automatically give you a greater right than anyone else to that domain name. Also, having a domain name in one domain zone (eg. "yourcompany.com.au") does not give you the right to stop others from holding the same or a similar domain name in another domain zone (eg. "yourcompany.com", "yourcompany.net.au", "yourcompany.co.uk", etc).
If you think you have a better entitlement to a .au domain name than the current registrant, you may be able to bring a complaint under the .au Dispute Resolution Policy (auDRP). Further details about the auDRP are available at http://www.auda.org.au/policies/audrp/.