Answered Dec 25, 2018
The terrain issues (flat vs. hilly) are certainly a big factor. It’s difficult to spread out horizontally in Wellington.
It’s also difficult to spread out vertically. The earthquake risk in Wellington dissuades the construction of high-rise apartment buildings like you can see in Auckland.
I would also suggest a social factor. A critical mass of immigrants. One person immigrates, and settles in a large city. After gaining citizenship, they then sponsor multiple relatives to also come over, and settle in the same local area.
Prospective immigrants from certain countries (China, India, Pacific Islands) may only have really heard of Auckland, and know that there are already large populations of immigrants from their same origin.
There are also certain cultural/ethnic/family issues, based on those origins.
According to the 2013 census, Pacific people have significantly more children than any other ethnic group. So, when you already have a lot of Pacific immigrants settling in one area (i.e. Auckland) they will then drive up the local population further with their high childbirth rate. And their NZ-born children and grandchildren will grow up, and likely continue that high childbirth rate.
Another issue could be the willingness/inclination of certain cultures (already present in Auckland) to have multi-generational households and extended family members crammed into the same run-down, overcrowded house. So they can tolerate an area with a housing shortage and high rents, without being financially driven out of the city.
The higher opportunity level and dynamic atmosphere of “the big city” may also attract people who already live in some other part of New Zealand. And, since Auckland is already the the biggest city, it may be first on the list of choices for many people.
My guess is that, Wellington will eventually become much larger (physically and in population), with a “supercity” extending the official city limits to swallow the areas up to Kapiti, Johnsonville, and Upper and Lower Hutt.
I’ve lived in Wellington previously, but am generally inclined to go to new places (Palmy is the the fifth [and smallest] city that I’ve lived in as an adult).
If I can line up a job there, I would move to Auckland without hesitation. (Yes, I know housing is expensive, but I have low standards).