This article was interesting Was Auckland the cradle of the Kiwi accent? An old theory gets a new hearing:

Why, for example, does our [NZ] accent have relatively little regional variation (other than Southlanders and their rolled Rs), despite many early centres such as Christchurch and Dunedin having distinct cultural identities?

But even more interesting was the embedded 18 minute radio interview with retired Linguistics Professor Elizabeth Gordon. Do listen.