Logically its when a vaccine is available and can be rolled out to the NZ population. There is a a massive international effort underway with over 100 in development and Trump making it clear the funds and resources available are unlimited, with manufacturing on mass of multiple candidates so that the right one is ready to go as and when it works. But when could that be rolled out accross NZ - at least a year from now absolute best, and that's if all goes really well.
But what does NZ do if there isn't a vaccine next year or the year after that? The global population slowly gets infected, but NZ is untouched. Maybe testing becomes super accurate, or some sort of immunity passport, or therapeutic treatment.
Personally, I think a viable vaccine and excellent treatments will be available this year. Or the virus might fizzle as some suspect now that it has infected the most vulnerable.
I think it would be extraordinarily unlikely that there wouldn't be a vaccine of some form of treatment, so New Zealand is banking on "eliminating" the virus now, so we can return to 90% normal, then wait it out, rather than get the virus and have if disrupt local commerce businesses due to everyone being afraid of going outside.
It's true it isn't guaranteed but given how many resources are being poured into it, I would be betting on it rather than not.
By betting on it, our domestic market is relatively back to normal. These past two days have been honestly almost back to normal from what I have seen. Obviously it's now modified and some stuff is a no go (large events) but it mostly feels normal now that we are in level 2. And we got to this stage super quick due to the "elimination" strategy.
Another country would be in the same position as us, but on a much bigger scale: China. Its barely gone through their population of 1 Billion!