Going by the "serious" documentary (most people simply send our accent up) hosted by John Clarke our accent is a deliberate creation in the 19 century by the freeborn "Currency" children of the convicts. The children of free settlers no doubt played a part in shaping the way we speak as well, but, it is the creation of children, all the same.
Convicts and settlers came from all over the British Isles bringing a hodgepodge of accents with them. The kids, according the documentary, wanted to separate themselves from their parents and say "no we are a product of this place, we were born HERE, not some place on the other side of the world."
So they deliberately got rid of any trace of the parents' accents in their own speech. The kids deliberately broadened and flattened their diction - and in the process of getting rid of "accent" created our distinctive sound.
This creates a trap for foreign actors trying to do an Australian accent, noted Rachael Griffiths on the tape, because they make the mistake of trying to "put it on," overlaying our accent with their own. If they focused on laying off their own accent, they'd have better luck. Australian actors on the other hand find it easy to do foreign accents.
The English often accuse us of being lazy or careless in our pronunciation. The "picture" in the wall has become a "pitcha" in spoken Australian:the drawn out "err" sound at the end becoming a short "ah."
....as a digression....
The situation with the English is a bit like the American Slave language of Gullah. The purpose was different, but to the slave owners it sounded like baby-talk and they shrugged it off as an example of the slaves' lower intelligence. Failing to understand Gullah as a separate enabled slaves to speak freely before their masters without being understood by them.