Learn the local language
Australians speak English, but they have their own slang words that heavily influence their conversational English. You can find yourself in the unusual predicament of speaking the language but not understanding what they're saying.
To assist you, here are some key words to remember: Togs/swimmers/bathers = bathing suit; tomato sauce = ketchup; "Do you want to join us for tea?" = "Do you want to join us for dinner?" Remember those and you'll be fine, but don't be afraid to ask questions; most Australians will gladly interpret for you, and you'll be proficient in no time.