The distinctive Australian accent is the result of a “drunken slur” caused by the heavy drinking of the early settlers, according to a communication expert from the country.
But the problem with this idea is that early Australia wasn’t notably more boozy than other societies of the time.
If I were to look for a cause (and as ever, I’m entirely willing to pontificate on something I know nothing at all about) I’d look to the rural/urban split. It’s well known that urban accents are faster than rural (just compare Cockney with Dorset, Boston with Maine). Oz was a very rural society for the important developmental part of its history. For that first century or so it was a giant farm for the Empire.