Some registrars use two passwords for their .au domains. One is sometimes referred to as a Registry Key and the other is a domain password (AuthInfo). To transfer a domain name you will need the Domain Password (AuthInfo).
Your registrar may have only issued you with one password, however it is sometimes the case that they issue you with a password compatible only with their internal systems.
If you have checked the password and it still does not work you will need to contact your existing registrar and ask for the AuthInfo Domain Password to use for a transfer.