Scientific experts say two or three key factors specific to Auckland's latest Covid-19 cases forced the Government to impose a seven-day lockdown.
The snap move applies from 6am today and follows confirmation of two new Covid community cases.
A man, 21, known as case M, went to the GP on Friday afternoon for a Covid test - then went to the gym. His positive test result was confirmed this afternoon. It has since been confirmed the man's mother has also tested positive.
Epidemiological and public health experts agreed the length of time the new Case M was exhibiting Covid-19 symptoms while mingling in public would have concerned Cabinet and the Ministry of Health.
University of Auckland Professor Shaun Hendy said Case M had shown symptoms since about Tuesday so might have been infectious all week.
"This may mean that there are downstream cases in the community that are also infectious.
"Secondly, although it is possible that the chain of infection leads directly back to the cluster of cases at the school, the school pupil in the family tested negative three times and has not had symptoms.
Hendy said although it was "not impossible" for someone to test negative three times, "it is unusual".
"Thus it is possible that the infection came via another route and that may mean there is another cluster of cases in the community, linked to the school or otherwise," Hendy said.
"Finally, assuming the whole genome sequencing does establish a link to the cluster at the high school, these new cases have shown us the limitations of our contact tracing system when people are not able or willing to cooperate.
University of Auckland School of Medical Sciences Associate Professor Siouxsie Wiles said Aucklanders would be surprised and shocked by Saturday night's lockdown announcement.
"The details of today's case are too concerning not to. Once again we have an unclear chain of transmission," Wiles said.
"We know that they are a sibling of a student at Papatoetoe High who has already returned three negative tests and has not had any symptoms. Though it would be highly unusual, it is still a possibility this could be the source of case M's infection.
"The serology testing may help shed some light on this as will the genome sequencing. We also have the fact that the case has been infectious in the community for as long as the last week and has visited a number of locations. Moving Auckland to alert level 3 and the rest of the country to alert level 2 puts us in the best position to get on top of this outbreak as quickly as possible."
University of Canterbury Professor Michael Plank said two key factors made Case M's situation so concerning.
"One is that there is no established link between the new case and the existing Papatoetoe cluster," Plank said
"The second is that the new case has likely been infectious since 21 February and has visited a number of popular locations in the community.
"[So] there's a high chance there are other cases we don't know yet about, and they have potentially been out in the community for some time."
Plank said it was "highly likely" these new cases are the more infectious B.1.1.7 variant, which means the outbreak has the potential to spread faster.
"For these reasons, a seven-day period at level 3 makes sense," Plank said.
"This will give our testing and contact tracing system the time they need to track down any extra cases and shut off chains of transmission. As frustrating as it is, this is the right move to keep Auckland and New Zealand safe."
Victoria University of Wellington clinical psychologist Dr Dougal Sutherland spoke of the mental considerations Aucklanders would have to contend with.
"'Be kind' has been the catchphrase of Covid-19. The Prime Minister said it, the media said it, everyone said it," Sutherland said.
"But kindness has its limits. Being kind takes extra effort, can be exhausting, even annoying, and it seems like it just lets people off the hook for all sorts of crimes and misdemeanours, like breaching the rules around alert levels for example. But if you're over kindness, consider trying compassion instead.
"The simple act of self-compassion can lift a whole lot of stress and pressure off your shoulders. And it makes it easier to find compassion for others: to recognise they stuff up, get it wrong or aren't as helpful as they should be."
The new case
The new case was a family member of a student from Papatoetoe High School. That student had tested negative 3 times before and had no known symptoms - that means there is no current link to the cluster.
The latest case, 21, developed symptoms on Tuesday.
He was tested on Friday and the positive result was received on Saturday.
Genome testing is under way - and there is a strong assumption that it's from the current cluster. But that cannot be confirmed yet.
Ardern said there was a "cause for concern" as the man has been infectious for a week and been to a number of "well-populated sites".
Cabinet met on Saturday evening after the new case was detected.
Director general of Health Ashley Bloomfield said testing of other household members has been completed.
The man's mother has also tested positive for Covid-19.
The new case was tested by a GP on Saturday but has been infectious since last weekend.
The new case works part-time at the airport.
More sequencing is under way and will be back later on Saturday - he said that would likely show a link to the current cluster.
Bloomfield said it was "very unlikely" that any of the tests from recent outbreak were false negatives.