Director of Public Health Dr Caroline McElnay has conceded that the testing regime so far does little to show the true extent of Covid-19 community transmission in New Zealand.
A new directive to test more and make criteria for testing looser would lead to more tests, an increase in confirmed cases, and a clearer picture of the prevalence of Covid-19 in communities, she said.
The growth rate in the number of new cases continued to decline, with the announcement today of 61 new confirmed and probable Covid-19 cases in New Zealand.
But while that was "encouraging", McElnay said it was early days and more cases will spring up as testing capacity increased to 5000 tests a day.
McElnay said there was still a strong link to overseas travel, with just over half of the cases reporting overseas travel.
About 1 per cent of cases were due to community transmission.
Community transmission the key factor in whether the lockdown will be lifted
McElnay said the 1 per cent figure was based on the case information available.
"There's a lot of uncertainty about that number because there is some information we still don't have on those case. It's what we can determine at the moment."
More testing would provide a clearer picture of community transmission, she said.
"In the next couple of weeks, we will have a much better assessment of what the community transmission is, bearing in mind we are already in a level 4 lockdown.
"That is a control intervention, so we could expect to see less community transmission."
Yesterday epidemiologist Sir David Skegg said the data was meaningless because the testing criteria was skewed to symptomatic people returning from overseas or those who have been in close contact with a confirmed case.
The actual number of cases was likely "far higher" than the Ministry of Health's numbers, he said.
He and expert epidemiologists including Professor Michael Baker have been calling for more testing for weeks.
The lockdown will only be lifted if community transmission has been stamped out, but making that call would be difficult without the testing to show where the virus is present.
McElnay said new criteria for testing would be issued today.
Anyone with respiratory symptoms consistent with Covid-19 should be tested regardless of whether they had been overseas or in close contact with a confirmed case.
"But we do rely on clinical judgment at all times," she said.
About 1843 tests per day were being conducted in the past week, with daily capacity at about 3700 tests.
The Ministry of Health had been in touch with 418 close contacts yesterday, McElnay said, but she had no data on whether that was more than previous contact-tracing numbers.
McElnay said personal protective equipment (PPE) was being distributed to healthcare workers as a priority.
"Certainly we're getting that PPE out to those on the frontline."
Some nurses have been asking for more masks, but McElnay said she did not have details about those reports.
"PPE is part of the controlled approach that includes physical distancing and hand hygiene."
Civil Defence director Stuart-Black the first charter flight had taken off to bring New Zealanders returning from overseas from Auckland to their homes around the country.
More flights would follow, and those people would have to self-isolate in their homes.
She did not have numbers for how many people coming back to New Zealand from overseas were self-isolating, or whether any of them had tested positive for coronavirus.
The national state of emergency was extended yesterday, and Stuart-Black said that was necessary because each time it is declared, it expired after seven days. The expiry was a safeguard, and states of national emergency can be extended as needed.
Stuart-Black said the emergency powers had been used a number of times so far, including stopping people doing non-essential activities.
She said most New Zealanders continue to comply with the lockdown rules.
Some road obstructions had been in place to keep neighbourhoods safe, and police had been providing advice, she said.
This week's jump in cases
The number of people in New Zealand confirmed with Covid-19 increased from 552 on Monday to 600 on Tuesday, with another 47 confirmed cases, while the number of clusters doubled to 14.
The largest new cluster is linked to the Royal New Zealand Air Force rugby team, which competed in a United States tournament last month.
Another new cluster was linked to a wedding in Bluff, which took place before large gatherings were banned in this country.
Yesterday Jacinda Ardern said she wanted more testing to be carried out, and Health Minister David Clark said this morning that 5000 tests per day would happen soon.
Yesterday about 1700 tests were done, even though there was capacity for about 3500 tests.
A total 367 deaths were reported in Britain on Tuesday, bringing its toll to 1651, while another 849 deaths pushed Spain's toll to 8189.
More than 800,000 cases of Covid-19 have now been confirmed across 178 countries, with Europe and the United States the new epicentres of the crisis. More than 38,000 people have died.