Oh, I'm Australian and I don't hate Americans at all! I want to move there one day - America is more my place to live than Australia ever was!
I think some Australians don't like Americans because Aussies like to stick to their own kind, as Australia is an isolated country. Americans think more like Irish, while the Australians try to be English or European. We all know the history between the Irish and the English.
In addition, Aussies find America's global presence "weird" and unknown - they probably can't get over the many achievements Americans have done, considering they are people with big ideas, and not God. Aussies find Americans too ambitious where they find it annoying and a threat to their comfortably racist ego and mundane lifestyle.