The fact it's an isolated island with a small population and it's their summer no doubt helps too...
NZ is highly connected, and it gets a significant number of tourists from China. Summer is the high season for tourism.
So if anything, NZ should be hit harder than many less connected states/countries.
And summer matters because most scientists think hot weather slows the virus' ability to spread.
NZ has temperate weather - it is not a hot country: https://weatherspark.com/m/144837/3/Average-Weather-in-March...
In March in Christchurch (approx 1/10th NZ population) the temperature would be called “cool”. I think you are rationalising.