The Health Ministry says tracing is vital but admits its new national tracing centre is "locked into" using only its own phone system at the moment, so cannot use staff elsewhere to do tracing.
It turned down an offer of help with contact tracing from a nurse in recent days because of this.
This comes as an Auckland high school says 800 girls are considered the close contacts of the four confirmed virus cases it now has.
Epidemiologists say mass, rapid contact tracing of people who have come into contact with virus carriers has been shown in Asia to be the difference between lockdown or no lockdown.
Otago University's Dr Ayesha Verrall said in Wuhan, China, 9000 people were in teams doing contact tracing; which per capita would work out at 4500 in New Zealand doing tracing.
New Zealand's national centre as of last night had 100 nurses, the ministry said.
The ministry said last Thursday all tracing would be done by a new specialised national centre it was setting up.
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This week, on Monday, before the shift to Alert Level 4 was announced, Director-General of Health Dr Ashley Bloomfield said tracing was being ramped up.
"We have anticipated this increase in cases and in addition to the capacity there is in all our public health units, we have stood up a team here in the ministry that is able to supplement the public health units and it can manage at the moment up to 50 new cases a day ... and is also scalable up," Bloomfield said.
"Our contact tracing efforts are incredibly important and they are being ramped up to support this public health response."
The ministry had put out a call for help from health professionals.
Danielle Davies responded - she had just got her nursing practice certificate reactivated urgently, within 12 hours, and contacted the ministry, which asked her last Thursday to start tracing work the next day.
"When I enquired how does work go about, they said that the shift would start in Wellington at 7 in the morning," Davies said.
"And when I informed them that I'd moved to Christchurch a few years ago, they said that, 'Oh never mind, we won't be able to have you be part of the Covid effort'."
She tried once more to sign up to help, at the weekend, but has not heard back.
The ministry told RNZ in a statement on Monday night why it could not use the nurse: "We welcome more help but at this stage the system we are operating is locked into using ministry phones.
"We are working to resolve this and hope to have a new system in place later this week to enable contact tracing remotely."
Last night the ministry updated its statement: "We are looking at cloud-based technology as an option and the new system will mean ministry phones do not have be exclusively used, therefore, remote working will be a possibility and the capacity for more contact tracing will improve."
However, "to date, regional public health staff have been doing the contact tracing", it said.
"As case numbers continue to rise, the ministry has added to supplement capacity and contact as many people as quickly as possible."
Staffing needs to be 'ramped up' - epidemiologist
This had worked with Covid-19 in South Korea, Singapore, Taiwan and in Wuhan, China.
"We will need a lot more than we currently have," Dr Verrall said.
"We certainly need to ramp up our staffing on the ground in public health units; our capacity in central call centres; and we also need to urgently look at technology that can make contact tracing almost instantaneous."
The ministry needed input from experts in scaling up tracing by the national centre, so that in a month's time, when the lockdown was reviewed, and if the alert level was reduced, New Zealand could do so with confidence that it would not let its guard down.
"We are nowhere near where we need to be," she said.
"We need to be coming out of lockdown with the capability to trace 1000 cases of contact a day."
Her colleague, Professor Philip Hill at Otago University, last week called for an urgent ramping up of contact tracing.
He declined an interview with RNZ saying he is now engaged with the ministry to come up with "novel approaches engaging people from multiple disciplines, to get ahead of the virus using the 'case contact management weapon'.''
RNZ understands the ministry engaged Professor Hill only yesterday.
The ministry said as of 3pm yesterday, 840 close contacts had been identified; 69 in the past 24 hours.
It did not provide the numbers of staff it has in its national tracing centre, when asked, or the number of health professionals who had offered to help.
"The number of staff continues to change as different locations ... have different case numbers that may go up and down," its statement said.
"A number of health professionals have joined the response around New Zealand as and where required."
It was not aware of any others like Davies who had been rebuffed.
When RNZ asked why the ministry did not suggest to Davies that she offer her help to Canterbury's regional public health unit instead, it said, "That would have been a useful suggestion.
"In this instance, unfortunately because of technology limitations we were not able to offer a remote position. Now things are progressing, we will in future be able to offer people the chance to remotely contact trace."
