The banks are required to stress test those loans with somewhat stringent requirements. When the loans were granted they take the current retail interest and add around 3% and ensure that the loan recipient could still afford it. Last year there were stories in the NZ media around how banks were not writing loans because people were spending too much on Uber Eats etc. They were going over finances with a fine-toothed comb (there was a bit of backlash in the media, but turns out it was a good thing). NZ banks have quite a high capital reserve buffer requirement which was increased by the Reserve Bank of New Zealand a few years ago – with a lot of pushback from the banks at the time.