'Australian' originates from the place-name 'Australia', ultimately from Latin 'australis', where 'australis' meant 'southern'.

'australis' in Latin passed into New Latin as 'Australis' and then into modern English place-name 'Australia'; the adjective and demonym 'Australian' developed from the place-name.

Initially it referred to the 'southern' (land); over time it became the name for the continent 'Australia' and then for people and things associated with that country ('Australian').