Interesting subject. I doubt all Australian accents really are the same, any more than all American, English, French or Indian accents are the same. There may well be similarities that mean that any Australian accent is easily identifiable as Australian.
Be that as it may, if you are interested in the subject, you need to understand the differences between accent and dialect, syntax and vocabulary. All these things combine to identify where someone comes from.
And someone’s origin isn’t just geographical. You get ethnic accents, class accents and so on.
It has been said that in England everyone is looked down on by someone else because of their accent. You get snobbery and inverse snobbery.
In the midlands and north, we tend to use a short a (grass rhymes with ass) and in the south east they tend to use a long a (grass rhymes with arse. In the south west and some other predominantly rural districts, the a is long and slightly drawled graarse.
In the north east, the vocabulary is influenced heavily by the Vikings. They have words like [I]hoy /I. In the south east, the language is more Saxon or Norman French in origin, so the vocabulary is different again. In the south west, there is Celtic influence - shown in the place names, but also in expressions.
Syntax: there are was towns and were towns. The “standard” English verb goes: I was, you were, he was, we were, you all were, they were. In some towns, they only say “was”. (I was, you was, we was…) and in in other towns they only say were, “I were, you were, he were…”
Then there are verb endings. “Standard” English has words like “broken”, “woken” etc. In Nottingham, the locals don’t say, “It’s broken” but “It’s broke.”
Interestingly, the “…en” ending is the older form. In America, they still say “gotten” although this died out centuries ago in England. A rare example of the Americans being conservative with our language. (America: the land that gave us disambiguate when clarify is so much simpler.)
So where do these accents come from? They are stronger in areas where there are few outside influences. Words develop or remain unchanged in a way that suits the local community, much like finches evolved for specific niches in Galapagos. As TV and radio have exposed us to a wider range of accents, changes are happening. In Nottingham and Derby, the traditional endearment is “duck” (Ey up mi duck = Hello my friend). The influence of the TV series Coronation Street (set in the north west) has introduced “chuck” as an alternative to “mi duck”. We now have Nottinghamians saying, “Ey up chuck.” (Shudder!)
I notice that as people grow more confident and gain education, they tend to soften their strong regional accents - and then as they approach retirement, they seem to readopt their original accent, almost self consciously, as if reasserting their identity as part of their home community. An accent can be a powerful “tribal” bonding force.