Dr Ayesha Verrall, of the University of Otago, wrote an independent report into the Ministry of Health's contact tracing system.

An independent report into contact tracing efforts has revealed the system was overloaded by fewer than 100 daily coronavirus cases.

The critical report into the public health measure, written by infectious disease physician Dr Ayesha Verrall, has found it was taking more than two days to trace Covid-19 cases contacts and advise them to self-isolate after New Zealand entered lockdown.

The report also revealed official efforts to build a cellphone app to help trace people's movements.

Contact tracing has become a critical part of the response to the spread of Covid-19, and there has been widespread concern it has lagged behind other measures such as border control and testing for the virus.

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Experts say a more effective system, which rapidly tracks down people who have come into contact with someone who tests positive for the virus, is vital for New Zealand to safely leave lockdown.

Prime Minister Jacinda Ardern, when extending a four-week lockdown of the country on Monday afternoon, announced the Government would spend a further $55 million on bolstering the contact tracing system.

The Ministry of Health released Verrrall's report shortly afterward.

In the report, Verrall said that during March the contact tracing system was overloaded by fewer than 100 daily Covid-19 cases.

"Many cases were returned travellers who had taken domestic flights, meaning the contact tracing workload was extremely high. As case numbers rose in March it became apparent that the workload would exceed the capacity of many PHUs," she said in the report.

ABIGAIL DOUGHERTY/STUFF A line of people spills onto the street waiting to enter the Countdown supermarket on Stoddard Rd in Mt Roskill, Auckland. Social distancing is a key response to the spread of coronavirus, and so is contact tracing for when people with Covid-19 come into contact with others. But New Zealand's contact tracing system might not be up to scratch to leave lockdown, experts have said. (file photo)

The ministry created a nationalised "hub" for contact tracing, called the National Close Contact Service, which started operating on March 24, the day before New Zealand entered lockdown.

Regional public health units, which have been managing contact tracing for their district health boards, were then able to refer a Covid-19 case's close contacts to staffers at the central hub for tracing.

By April 6, 200 staff had been trained with a new contact tracing system which was linked up with contact details held in the National Health Index.

During this first two weeks of lockdown, the average time between identifying a positive Covid-19 case and telling the person's contacts to self-isolate was 2.3 days.

"The timeliness of the process was poor," Verrall said.

"Only 60% of contacts could be easily reached by phone, either because of incorrect contact details or because people choose not to answer calls from an unidentified number.

"Linkages between the National Health Index and other health datasets were being established to address the first problem."

She said staff at regional public health units were cautious about referring tracing work to the central hub, as they could not access their contact tracing system to see how quickly the contacts had been reached.

Verrall said to adequately bolster contact tracing, the regional health units - which could only send some of their tracing work to the central hub - would have to be better resourced.

"It is highly likely that there will be multiple instances of community transmission needing case management and contact tracing at intervals and across the country for the next year and beyond.

"To avoid regular nationwide returns to level 4 restrictions, [regional public health unit] capacity must be increased ... [they] need the capacity to confidently manage cases and clusters through a combination of case isolation, contact tracing and potentially targeted restrictions on movement."

Verrall said a smartphone app was being built by the ministry and local developers, who she did not name, but she provided no further detail as the app was still being considered.

She said any app would need to be quickly developed and piloted, and taken up by a large number of New Zealanders to be effective.

HAGEN HOPKINS/STUFF Director-general of health Dr Ashley Bloomfield has said New Zealand was some days away from having a "gold standard" contact tracing system. (file photo)

There was potential to use both an app which recorded people's close contacts through Bluetooth detection of another phone, and an app which used QR-codes to scan people into public transport or cafes, tracing their movements.

The ministry on Monday confirmed it was working on a range of products that would be "useful for helping to ensure that we fill in the gaps in people's memory, and identify close contacts whom the positive person may not know".

Director-general Dr Ashley Bloomfield, who commissioned the report from Verrall, said efforts to scale up and nationalise the tracing system continued "apace'.

The "gold standard" of contact tracing - the ability to trace 80 per cent of a person's contacts within three days - has been described by Bloomfield as being days away.

Bloomfield said Verrall was "complimentary" of the work being done to improve the system, and had only identified small gaps.

The national contact tracing centre could now trace up to 5000 contacts each day, Bloomfield said.

"We're going to continue to build our capacity, what we're wanting to be able to do is to scale up even more rapidly if we need to."

Ardern said it remained important to have contact tracing staff with "local knowledge" of their communities, hence regional public health units would be funded with the $55 million boost, and a "surge capacity" of another hundred full time staff would be added.

ACT leader David Seymour said the Government had failed to sufficiently build contact tracing capacity, and this was the reason why the lockdown was extended.

"The delay is really because the Government has failed to bring its contact tracing abilities up to an adequate standard. It has nothing to do with extra certainty, because there's no indication that this decision could change if our test results deteriorate between now and Monday," he said.

National leader Simon Bridges similarly attacked the Government's efforts to lift the lockdown, saying the contact tracing system had major shortcomings.

The "groundwork" had not been done, and businesses would suffer, he said.

ROBUST TRACING NEEDED

The opportunity for the virus to spread will grow as people leave their homes and their contacts with others multiply, which has led experts to call for a more effective contact tracing system.

Sir David Skegg, an epidemiologist who has recently appeared at Parliament's epidemic response select committee, has said contact tracing could be the "Achilles' heel" in the coronavirus response.

The tracing system needed a "tremendously improved" capacity before lockdown ends, he told MPs.

Professor Michael Baker, an epidemiologist at the University of Otago, has with colleagues including Verrall published a paper which describes contact tracing as essential for "successful, sustained elimination" of Covid-19.

"Such [tracing] systems need to be supported by effective use of digital technologies, including mobile phones," the academics said.

In a paper titled The Future is Now: Implications of Covid-19 for New Zealand, Sir Peter Gluckman and his University of Auckland colleagues described existing contact tracing efforts - as well as surveillance testing in the community - as "far from ideal".

"Contact tracing is fundamental to breaking infectious disease pandemics and will be even more important once we reach [lower alert levels] to reduce the risk of a major flare-up," the paper reads.

Gluckman, the prime minister's former science advisor, has said manual contract tracing would not be enough if if there was a "blowout" of the virus during the coming winter.

"People don't want to be yo-yoing in and out of lockdown, or high levels versus low levels. People have made large sacrifices ... We owe it to all of them and to our safety to have all the weapons at our hands that will keep us safe," he told Stuff earlier this month.