Yeah it's a much easier problem when you are on a tiny island where you can completely control travel. They could shut down again at any time though, as Australia just did. I don't know if New Zealand gyms are open agin or not, but it's pretty risky since they have such little immunity built up. A single positive case that gets through the border could easily lead to widespread outbreaks if social contact levels within the country are at nominal levels.

But yes, as long as you can completely control travel into a geographic area, then quarantining the virus away makes sense. New Zealand recently had two positive cases that could have easily led to further widespread outbreaks if not caught at the border. I do not see how this level of travel restriction is feasible for the US. New Zealand is very likely to have more outbreaks in the future as well. They have to walk on eggshells for the next year at least, as it only takes one mistake to start the whole thing over.