I don't think there is only one Australian accent!! yet
I found similarities between London's Eastenders 'ei' diphtongs and Australian speakers(informants) I overheard just like Peter Garret of the late Midnight Oil
ei--> close to ai
I think you'll find that there is no accent in the UK that is the same as an Australian accent.
The First Fleeters came mainly from around east London, but that was over 200 years ago.
If there is any similarity to the accent of Manchester, that would be a complete coincidence since there is no special historical connection.