More likely the early settlers, being transported convicts, were the dregs of British society whose accents and manners of speech were the absolute worst of the worst.
Brits wanted us to believe it is true, but it is not completely true as many documents that surfaced in the last 15-20 years demonstrate.
In reality, only small percentage were criminals transpoterd Down Under after U.S. got independence (Georia was British penal colony until then). The rest were either poor people forced to move or the children of London's tens of thousands of prostitutes.