To change the legal owner of a .au domain name please visit:
https://www.enetica.com.au/register.cgi?action=reg_transfer
You will need written authorisation from the existing domain owner and a transfer fee of A$189 is applicable.
Please call Enetica on 1300 720 043 or +61 2 9779 1166 once you have sent the fax as advised on the form at the above link.