It's been described as the "critical piece of information" health officials would look at when suggesting whether New Zealand is ready to move to alert level 1.

But when will we know for sure that community transmission is no longer a hidden risk?

It's been 46 days since the country had a case that couldn't be linked to a known one.

While there have been nearly 1500 confirmed or probable cases to date, the number of active cases now stands at just 47 – and five out of the last seven days have seen zero new ones.

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Today, director general of health Dr Ashley Bloomfield said New Zealand was now in a position where there were no cases of community transmission – but that didn't mean the danger was gone.

While he wasn't concerned about any spike in cases since New Zealand moved to level 2, he was watchful.

"Obviously we want to remain watchful to keep up that testing across the country to make sure that if there are any cases that may have gone undetected, we are finding them," he said.

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"And we are more likely to find them or see a spike because people are interacting more – so hence continuing the wide testing, and being ready to act very quickly."

As officials reviewed person limits on private and public gatherings, they'd want to still see that there were no cases that couldn't be linked to existing ones, he said.

"My sense is that New Zealanders want to make sure we capture the gains that we made through alert levels 4 and 3 – and we are not talking about months before we are considering the numbers – it's being considered again in a week's time."

Bloomfield pointed out the danger of Covid-19's long incubation period; it could take two weeks for a person infected to begin showing symptoms.

But, he noted: "The work we have put in over the last seven weeks, particularly through the lockdown and our stepped and managed approach to come out of the lockdown, has put us in the best possible position.

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"And we can now have a high degree of confidence that there are not cases of Covid-19 out there that are creating risk."

Otago University epidemiologist Dr Ayesha Verrall agreed that unexpected community transmission would be a crucial indicator.

"It reflects the fact that we don't have control, if we have one of those cases," she said.

"The idea of this whole thing is looking for the needle in the haystack – that one case still out there – and my view is the probability of finding it is greater if you test in the pool of symptomatic people."

As for when we could be confident we'd stamped out that risk, Verrall pointed out that New Zealand still had active Covid-19 cases.

She added we could be surer when the statistical probability of undetected cases reduced over time, all while we carried on testing at a high rate.

But there wasn't any statistical threshold at which we could categorically state the threat had been removed.

"We need to get a group of experts together and make a surveillance plan that will answer that question - and set exactly where that threshold is."

New Zealand's ultimate goal remains to eliminate the virus and modelling has suggested that's possible if the average number of people directly infected by a single infectious individual is pushed down to one.

At the same time, researchers have pointed out there's currently no accepted international definition of Covid-19 elimination.

For New Zealand, one group of researchers have proposed it should mean there have been no new cases for a specified period, such as 28 days, and that a high-performing national surveillance system was testing a certain number of people per day, across the country.

Covid19.govt.nz: The Government's official Covid-19 advisory website