New Zealand has 5 million inhabitants. That would mean these other countries would need to have billions of people (2.5-5B people). I suspect you just missed a decimal when looking up the number.

Being an island and having a government that doesn't openly reject science certainly helps but they still needed to actually get it done. Which they did. Now they are in the comfortable situation that they can quarantine visitors and sit tight until there is a long term solution. Meanwhile their economy can function pretty normally.