Your question is a huge generalisation, first of all because there is no single 'Australian accent' in the same way as there is no single English accent or American accent. To my English ears, the Australian accent, especially as spoken by people from Sydney, actually sounds like something pretty much midway between a standard Southern English accent and an American accent. Probably the reason you perceive the Australian accent to be closer to the English accent is because both standard accents experience non-rhoticity, ie. the r at the end of the word car isn't pronounced. This is something that developed around the late 18th/early 19th Century around London, which meant that the US, which was settled much earlier, still kept rhoticity (except in New England) while Australia, which was only just beginning to be settled, exhibited non-rhoticity.