New Zealand is too far away from the rest of the world, and has been populated for too short of a time for it to be able to reach the UK's Population. My brief search says New Zealand hasn't even reached England's Population in 1700.
Yep. NZ
can support 60 million people, but that many people simply wouldn't be present, without some sort of stimulus to boost migration to NZ.
Two ideas for how this could potentially happen:
One, a little cliché: Nazi Germany wins WWII, demands New Zealand off the UK. It then gets turned into a huge squalid deportation camp for the "Untermensch".
A better idea, and more interesting:
In the American Revolutionary War, the entire British America (including Canada) becomes part of the USA. With efforts in America destroyed, Britain focuses its attention to Oceania and India. The new city of Auckland becomes the main base for Britain's operations in the Pacific. Britain encourages colonisation and exploitation of New Zealand.
The British King decides to temporarily relocate to Auckland, under constant fear of an invasion by Napoleon. Several wealthy landowners follow.
Napoleon focuses on eliminating his main rival the UK, before tackling the continent. After a careful ambush, the French manage to take over the UK and capture its navy.
The British Empire in Auckland is severely weakened, with only a few million people and no capability to retake the British Isles. With the ferocious wave of nationalistic hatred of the French in the remnant British Empire, Britain vows to retake the home isles one day given the power.
A program is initiated whereby women are strongly incentivised to have as many children as possible, to restore the population available for recruitment into the Navy.
In Europe, Napoleon solidifies his control of Western Europe. Liberation looks unlikely for now.
In the British Empire (Oceania), fertility rates soar. A hundred years later, New Zealand is home to forty million people.