"New Zealand accent, aside from the short "i" sound, to outsiders is more or less the same," I said. "Although some Australians pronounce "plant" with a sharp "a" like in "ant"."
"Oh, they must have been influenced by America," my friend Colin said.
"Nah, probably influenced by somewhere in England," I remarked and perhaps is a consolidation of my whole insight into dialects and accents.
I'm not a linguistics professor, nor a historian. But slow learning languages and accents themselves, noticing patterns and looking at history have provided me with my own conclusions which I'd like to elaborate here. I'd like to raise a book read aaaagees ago which is worth a read, not for facts but the kind of thinking that can really blow you out of established facts about language evolution. It is The Secret History of the English Language by M J Harper. One of it's simple posits, recalled because I don't have it as a reference right now, is that language is because that's how it was. Evolution, whether biological or linguistic, isn't fast. Even if someone in an ivory tower evolves language, like a Shakespeare or a Bacon, it won't cause the majority of the population to develop a different way of speaking. There is huge buffering against change. That's why, as I'd agree, Middle English didn't evolve out of Old English (as the book stridently puts forward). A language basically like Middle English (think Chaucer) probably came to the British Isles. It probably wasn't the language of the ruling class early on, that must have belonged to another tribe that would be relegated to history. No-one would say the people who wrote in Latin spoke like the early Romans, just as no known region spoke in Classical Chinese, which existed right up to the twentieth century as a means for communication.
And so it was from my experience with Cantonese. It might seem a mystery that there are different accents in different towns just kilometres down the road, but simple when you look at language not as a monolith but as a mass of diverging, interacting units. They're not unlike biological evolutionary units. We're not surprised to see two birds, a kakapo and a kiwi, living on the stretch of land (let alone a sparrow if it should hop by). And it would be a mistake to think that these two derived from an ancestor that once existed on the same strip of land. Such is it with Guangzhou Cantonese and Qingyuan Cantonese. Qingyuan is probably not a Guangzhou colony, but a place settled during the same period of migration by a bunch of different clans. Chinese clans are patrilineal, so I'm currently in a patch of at least two settling families, both called Zhong. Our Zhong family isn't actually Cantonese but naturalised Hakka. The other Zhong family is local Cantonese. By proximity, they speak a similar brand of Cantonese. Women have been married into other towns for millenia but a single woman here and there won't make much difference to the linguistic core and development trend. So probably not in the distant past different families brought their tongue as is to this strip of land in the Pearl River Delta. The fact that they're somewhat similar, mutually understandable, indicates they were part of a similar region from their originating area (the central plains of China). Biologically they can still interbreed.
Hakka dialect can't interbreed in terms of pronunciation or grammar because it's too different. You would learn both separately and choose one to use. They're mutually exclusive for communication purposes. Hakka was a much later mass migration to Guangzhou. The logic is the same but much clearer for Hakka because they're more widely spread: it's hard to imagine them as a single clan. They were another snapshot of central plain Chinese language and culture that spread south in a wave. The wave is important because isolated families that move are unlikely to retain their language. They'd need to assimilate to marry their women out and make alliances. Where there is a wave of migration, there'll be enough other families to support the continuation of the culture and language without the need to rely on others, who won't necessarily give up their own dialect or culture for the sake of others.
And if that weren't incomprehensible enough, I'd like to return to our Ozzie friends and their "plant". My friend was quite rational in his guesses of American influence, but it can't be right. The fact that Americans, almost all Americans, use this pronunciation, indicates that some of the colonisers undoubtedly said it that way. The United Kingdom has all sorts of accents there. Immigrant countries have a lingual lottery from the proportion of peoples from various regions. If the population is small enough, they average themselves out to make a core identifiable accent that is a mean of all the parts, more or less. Australia, in some regions, probably have people from the same areas as the American forefathers who brought the same sound over.
This could be all claptrap, but it's the kind of claptrap I'm glad I have a working theory for. Such things are some of those dirty secrets of the way the world works.