How did Aussie slang begin??

Answer 1

It's been influenced by slang from all over the world, but especially Cockney (via the convicts and working-class settlers), other English dialects (and, to a lesser extent, Scottish and Irish), and a fair bit of Aboriginal vocabulary. A lot of slang terms started off amid specific occupations (e.g. sheep-shearing) and then made their way into everyday speech.

Answer 2

I'm sure it has it's roots in British slang.