You can't make any conclusions on the actual dates the border was closed - it's about what happened prior to that and how is situation developing in any given case.
This is what NZ did (Ministry of Health, not PM):
https://www.stuff.co.nz/national/he...irus-understanding-new-zealands-covid19-curve
Due to immense difference in circumstances (size, numbers, etc...), I argue that US should have done much more, much earlier and actually tried to enforce some of the measures. Would it work? Who knows, probably not but it might have given people a fighting chance by slowing down a spread. Lot of people still don't realise the seriousness of the situation.
And for the record - I certainly do not put NZ PM on a pedestal and repeat - I wanted a lockdown earlier. And I never voted Labour / left in my life but am sure as hell thankful they happened to be in government right now. I have a hunch National party's priorities would have been different.