The Opposition has taken to pouring doubt on the Health Ministry's assertion it can trace 5000 people a day who have had contact with a Covid-19 carrier.
Director-General of Health Dr Ashley Bloomfield said this is the current capacity.
An independent audit by Dr Ayesha Verrall said that tracing was too slow and constrained.
The 5000 maximum would increase as resources were added, Dr Bloomfield said.
The government said it will put $55 million more into the system.
In early April, the national centre averaged 450 contacts traced per day over a four-day period, with a high of 700 on one of those days.
National Party's health spokesperson Michael Woodhouse said it was not believable to think that capacity had risen so fast.
"There are 220 people in the contact tracing centre, and just 100 in the public health units.
"They're doing a number of things, not just contact tracing.
"So it's a mathematical impossibility for 5000 successful contacts to take place."
Factored in too, was the number of calls that failed because no one picked up or the number was out of date, which the audit shows had been running at 40 percent of calls made.
"Now unless they're able to fix that, the government won't be able to have the confidence that they can lift out of level 3."
Dr Bloomfield told Morning Report that 220 trained staff had helped supplement those in public health units (PHUs) to build capacity, meaning there was more than 300 staff who could be deployed to make close contact calls.
He said "the math works" out, and it would come out to be about 18 phone calls a day for each staff member.
"We have scaled up quite explicitly to get to that 5000. "
The ministry told RNZ in a statement on Friday that two-thirds of the contact details of all New Zealanders in its databases were correct, but one third "may be out of date", and it was still trying to find technology to fix that.
At yesterday's briefing, however, Dr Bloomfield said well over 90 percent of people's contact details in the National Health Index were correct.
In the first 10 days or so of tracing since the national centre began on 24 March, out of 13,000 calls made in total, only 4000 or so contacts were found. The centre has added a few dozen staff since then.
The audit said the fast establishment of the national centre was "impressive". However, it found the public health units were the weak link, under great strain, with too few staff and needing an urgent boost to tracing capacity of 3-to-4-fold.
The audit was damning, but Dr Bloomfield had denied to the Epidemic Response Committee as recently as a week ago that there was "even a problem with resource", Woodhouse said.
Committee minutes show the Director-General said "we're certainly coping with the current" case numbers, though new cases by this time were low. He told the committee they wanted to be able to ramp tracing up, and were "well on the way" to being able to trace the contacts of several hundred cases a day.
The committee was told by Australia's head of public health that it had about 7000 staff doing contact tracing; that is about five times more per capita than New Zealand.
Dr Bloomfield also told the committee: "One of the nice things that Dr Verrall points out is that, actually, really good contact tracing can be as effective as a vaccine."
Late last week, the epidemiologist on the ministry's advisory group, Michael Baker, told RNZ that he had never seen the data on public health unit capacity, and doubted it existed in a readily analysable form.
This was confirmed by Dr Bloomfield on Sunday, when he said the analysis of the public health unit data had been held up by creaky localised information systems, so analysis of it had had to wait till the last minute at the weekend, in order to provide it to the Cabinet yesterday as part of its alert level deliberations.
The audit said the ministry should "rapidly develop" a smartphone app to help with tracing.
"It needs to be available quickly," Dr Verrall said.
RNZ understands that Auckland company Rush Digital is building the app for the ministry.
The ministry has not confirmed this, or said how it gave out this contract, despite repeated RNZ questions.
RNZ also understands that a Bluetooth-based tracing app, like Singapore's, might be delayed for use here till late May.
Dr Verrall discusses contact tracing system weaknesses
Dr Verrall told Morning Report in hindsight it will be more noticeable how biased the pandemic plan was towards other diseases like influenza, where contact tracing didn't play a key role.
"The crucial thing we realised in time in New Zealand is that it is important [to contact trace] and we've been able to scale up, and when you look at situations in europe, UK, USA, I think that realisation never happened."
However, Dr Bloomfield defended the plan, saying that all countries had to pivot their response to deal with this outbreak.
"We didn't go from classic phases of 'keep it out, stamp it out', then manage it, we decided we're not going to be able to manage it because we saw what happened in other countries, so we had to adjust our response. I think we did that really quickly and really effectively, our contact tracing is usually out in public health units and dealing with local outbreaks.
"The fact that we've been able to scale that up actually in a really short period of time I think is impressive and that's part of the reason that other countries are looking to us and our efforts."
Dr Verrall said one of the weaknesses in the system was that every Public Health Unit had its own contact management system.
"We have a single case data system for New Zealand but for the [contact management system], everyone built their own, some of them are really sophisticated, and others are really basic so there's no ability to track contacts if the outbreaks spreads across different health units.
"And when the contacts are referred to the national close contact service, the public health units lose visibility of them and that means if they are not followed up in a timely way, there could be an outbreak developing in your region ... but you wouldn't know about it.
"Those information systems that underpin our contact tracing efforts are really crucial to be able to respond quickly to Covid-19 and to track the quality of our response."
She said if they systems built to the capacity outlined in the report then we would be resilient to small outbreaks and clusters. The report also highlighted a number of key performance indicators to define good contact tracing - including timeliness, equity and completeness.
"They need to develop a software system to allow them to capture that data and allow them to be able to report it in real time, that's how New Zealanders can be assured that we've got a system that'll keep us safe."
In response to the report's reference to build capacity to be able to deal with a cluster of 1000 people, Dr Bloomfield said there was no particular science behind that figure, and it was more of an aspirational one.
He said Dr Verall had referenced a church in South Korea for that figure, which had 4500 infections over two and a half weeks, averaging about a couple of hundred cases per day.
Dr Bloomfield said they would be able to further scale up another 300-500.
"Remembering that contact tracing is one part, but it's heavily reliant on the first interview that is done when you've identified a case ... and that requires a lot effort to go through with each person, what their movements have been in the previous week, so that's the capacity that we need to build and that will involve us working with the PHU which is the work we've got happening apace in the next week."
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