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.