Do all Australians speak with an Australian accent?
Australian English is spoken by people who are born in Australia and /or grow up with an Australian English speaking peer group. These people come from a range of different ancestries. However, there are of course many Australians who do not use an Australian English accent, either because they have lived overseas for many years or because they migrated to Australia at an age when their accent has already become relatively stabilised. It is true that adults can, and indeed do, change their accents, however early exposure to a dialect is the usual mechanism for acquiring a native like accent.