The stopped non-residents from entering New Zealand from infected areas.
They required residents and citizens coming from infected areas to do mandatory quarantine.
Implied: They had low travel volume (being disconnected helps).
The used contact tracing to find and isolate contacts of known cases (recursively).
“Prime Minister Ardern credits her country’s approach of “going hard and going early” as central to its success in containing the COVID-19 epidemic.”
They also have more sheep than people (and sheep don’t catch SARS-CoV-2).