The most Australian sentence going is 'throw a shrimp on the barbie', the cheerful invitation every Aussie supposedly extends at a backyard barbecue. Foreigners trot it out the moment they meet an Australian, certain it is authentic local slang.
No Australian says shrimp; we say prawns. The famous line comes from a 1984 advertising campaign aimed at American tourists, fronted by Paul Hogan, who said 'I'll slip an extra shrimp on the barbie for you'. The word 'shrimp' was deliberately swapped in because Americans call them shrimp and would not have understood 'prawn', so the slogan was tailored for a US audience, not lifted from Australian speech. In Australia the creatures on the grill are prawns, full stop, and have always been. The ad was a huge success and burned the phrase into the global idea of Australia, which is why visitors keep saying it to bemused locals who would never use the word. So the quintessential Aussie catchphrase is an American marketing line that no Australian actually says. Throw a prawn on the barbie, by all means. Just know the shrimp was for the tourists.