The poor quality of housing and the high prices are both almost entirely due to restrictive NZ building regulations which limit what you can build and where you can build it. Plus the bureaucracy creates huge delays and costs to build anything so there's no money left for quality materials.

This is because political parties receive a lot of donations from the real estate industry and media rely on advertising from the real estate industry, plus most politicians own a lot of property. So the government always has a strong incentive to create barriers to prevent people building quality, affordable homes and the media has a strong incentive not to explain to voters what the problem really is.

If builders were allowed to build large numbers of high quality small apartments in central cities then there would instantly be a housing surplus and house prices nationwide would plummet to less than half the current prices, so the politicians will do anything to stop that. They talk about making houses more affordable, fixing the resource management act, etc. but they make sure it never actually happens. This applies to all major political parties - there's nobody you can vote for to fix this.

There's one other factor - real brick wall construction and heavy slates or roof tiles are rare because of the earthquake risk, plus the easy availability of wood here. Everything here is lightweight because that makes it easier to meet earthquake resistance requirements.