Show your Australian passport (assuming you entered the country on one) to immigration/border force. They will check that you have been out of the country for the majority of the last 12 months.

Anyone who is normally a resident of a country other than Australia (i.e. tourists) I'm an Australian citizen who flew out of Melbourne on 29th March to the UK, where I'm normally resident. At the airport an immigration officer explained to me that the definition of "not resident" was anyone who...