An amazing amount. Australian English follows most of the rules, styles and spellings used in British English. The Australian accent is also often deep, warm and relaxed. Despite the differences, we receive a fair amount of American English in our media (music, TV, films) so the intricacies of American English are also pretty familiar to us. :)

What the Peachey guy said is pretty much it. If you can understand British English fairly well, Australian English shouldn't be a problem at all, aside from a few slang words here and there which shouldn't be that hard to pick up. One example regarding pronunciation is that Australians and New Zealanders, like the British, usually don't pronounce the "r" at the end of a lot of words, whereas in America and Canada we do. 很象南方人很少说那个北方的”儿化“的样子，你觉得是类似的吗？ I think most educated Americans are also pretty familiar with the Australian as well as the British accent. In regards to your other question about the phrase "hit me up" it just means initiate a conversation with me when you feel like it or you want to talk about something. Sorry I haven't been on this thing for a while..