Accents tend to take a long time to develop and the more isolated the region the bigger the tendency to be different. Relatively speaking Australia is quite young and although geographically isolated modern transport and technology from the 1900's onwards has meant that the isolation is lessened.
Australia has less variation in accent due to the fact that the majority of people who came here in the early years were from England and Ireland whereas The US had a far bigger melting pot of nationalities.
In saying that if you listen to people from different regions in Australia you can make out certain differences.
Adelaide and South Australia tend to pronounce there A's as ah rather than eh - the way they pronounce dance is 'Dahnce' whereas in the eastern states its 'Daance"
Tasmanians always sound different as do North Queenslanders, who often end their sentences in a higher tone. Even Sydney and Melbourne have nuances, particularly how both pronounce 'castle' differently.
NZ get their different accents due to the fact that they had a lot of welsh and scottish settlers, Canada is influenced by the French language.
I think this sums it up pretty well.
However, there are regional differeneces, and even though it can be difficult to isolate what those differences are, when someone is speaking, it is often possible to identify where the speaker is from.
When I lived in Tasmania, i noticed that many people pronounced the word "girl" in a very "English" way; very short and clipped, like "geh'll", whereas I would tend to pronounce it with an extended vowel, like "gurl".
I think it is also interesting how accents change over time. Listen to the audio from some old newsreel footage from the '40s. Contrast that with the way many young people (<20s) speak. I think there is definitely an increasing American influence on the Australian accent, and a shift away from British. But I think we still manage to put some Aussie "spin" to the language. Its all good
