People don't want to live in the South Island. This is actually causing problems in terms of NZ's proportional representation system because the law fixes the number of South Island electorates at 16, but it grows so much slower than Auckland that we keep adding new electorates in Auckland (and, possibly, some other places in the North Island). However, the total number of MPs remains fixed, which means that the number of list seats that are apportioned to maintain proportionality is decreasing... which is bad.^1

Now, you're probably really asking "why don't people want to live in the South Island?" but population growth is a snowball... people want to move to places where people live already. A context which is missing here is that Auckland (even allowing for two years of population loss) has more than a third of New Zealand's entire population.

^1 Before the special votes were counted, at the last general election NZ nearly had a proportional representation system that returned a parliament where a party had more than 50% of the seats with less than 50% of the vote. Though I personally think that was more due to all the votes that got thrown out on account of being for parties that won less than 5% of the vote (this figure was historically high).