Accents come from heritage and where you grow up.
The reason why the australian accent sounds like British, is because some time ago, Australia was a type of prison for British criminals. That is the reason why. Now, there is no real american accent. America is so huge, the diversity is bigger. There is: New Jersey, Tennesee, Texas, and Californian accents that I can think of. These accents have been formed by the many ancestors whom immigrated there.
EDIT:
Ex.
Irish + British = Tennessee (Hillbilly [No offense])?
Tennessee + Native American = Texas?
? = Not sure