Domain names are registered on a first-come, first-served basis. That is, as long as the registrant of the domain is eligible to hold the licence (if they meet all the requirements), then they can keep that name.
You can attempt to contact the owner of the domain name licence, enquiring whether they wish to transfer it to you, however they are under no obligation to do so.
Please note, the sale of .au domain names between parties is prohibited and any person found to be selling a .au domain name will have their domain licence revoked by auDA. All other domain names can be freely sold between parties.
