If you are you still in contact with the previous owner of the business, then you can contact them to assist in the Registrant Transfer process. You will need to contact your registration services provider for the form.
Otherwise, you can submit a formal complaint using the form available at http://www.auda.org.au/industrycomplaint/.
If you have a deed of sale that explicitly mentions the domain name and that the domain name licence was to be transferred to you, as the new owner of the business, you can submit that as evidence once you have registered the complaint. Documents can be emailed to email@example.com.
If you do not have a deed of sale, it does make the situation very difficult to resolve. Should this be the case, we will assist where we can to resolve the matter.