They will be testing, but as the incubation period can extend to two weeks, quarantine for 14 days will still be needed.
If these travellers (who wished to attend a deathbed, so couldn’t wait IYSWIM) had tested negative after partial quarantine and been allowed to make that limited compassionate visit, it’s wholly in keeping with a quarantine and test on exit strategy. But the incubating virus cannot be stopped without full 14 day quarantine.
So yes, NZ will have to be remarkably vigilant and have ‘no exceptions’ quarantine. That is a policy with both economic and social consequences, and I guess only palatable in remote places which have sea borders and are reasonably self sufficient.
Herd immunity, whether by vaccine or disease, is the only possible way of making it liveable-with for more connected parts of the world. And until we know more about how immunity to this virus actually works and for how long it can be relied on, we cannot know if that end state will ever be achievable