Australians officially speak British English, and Americans often mistake Australian accents for British accents, but we usually use prepositions and structure sentences just like Americans do. If you have trouble understanding a British accent, you'll probably also have trouble understanding an Australian accent. An American friend told me that I sound British when I speak slowly, but I sound like I have an accent from Boston when I speak quickly. We spell words like the British do, but words with American spelling are considered 'acceptable variants'. One of the major Australian political parties is called "The Australian Labor Party". "Labor" is usually spelled like "labour", but for some reason they use the American spelling. There's no grammatical difference as far as I'm aware, although I often feel that Americans deviate much further away from 'proper' grammar than Australians do when speaking casually, but we can also sometimes be very lazy. Of course, some people in both countries speak much more properly than others. In terms of vocabulary, we use a mix of British and American vocabulary (where there are differences). We use all of "rubber" and "eraser", "pub" and "bar", "mail" and "post", "diaper" and "nappy", "highway" and "motorway", "lift" and "elevator", "chips" and "fries" etc. We use "college" like the British do, "corn" instead of "maize" like the Americans do, "holiday" instead of "vacation" like the British do, "autumn" instead of "fall" like the British do", "potato chips" instead of "crisps" like the Americans do etc.

Part 2: There are also some words that exist in Australian slang that don't exist in either British or American English, but not nearly as many as you'd think. Everything that I've ever written on italki has been in "Australian English" ie. I write exactly how I speak. So if you can understand what I've written, you should have no problems understanding Australians in general (after getting used to the accent). If you come to Australia, you WILL hear slang that you may not understand (mainly among teenagers and young adults), but not so often that you won't be able to guess definitions from the context.

The most obvious difference is in the accent, though there are many different American (and probably Australian) accents. Australians draw out their vowels more, and accentuate certain sounds, such as "r"s at the ends of words, more. The other difference is that words may be different between the two. Australian English has more in common with British English than American English does. Here is a list of differences between them. http://www.aussieontheroad.com/lost-in-translation/

well, there are different words for nouns. That is going to be true between any two styles of English: Brish - American, Aussie - American, American - Canadian, etc. Then they have a characteristic pattern of vowel changing. A -> I, (mate sounds like mite), don't pronounce an R at end of a word. I am not australian, so better for you to see a video on it. http://www.youtube.com/watch?feature=player_embedded&v=dG0v9tZStAk

http://www.youtube.com/watch?v=jdroRRo2Z1w This is an Australian comedy making fun of an Australian news show. The comedians and the news reporter both have the exact same accent, but the reporter is very well spoken while the comedians speak much more casually. Can you hear a difference?