A border incursion every two weeks and several subsequent cases, many of which remain unresolved - is another large outbreak inevitable? Derek Cheng takes a deep dive into what's going on, and whether we should be alarmed.
Cabinet Minister Ayesha Verrall wipes her nose with the back of her hand, not because she has the sniffles, but to demonstrate how easy it can be to spread Covid-19.
The Weekend Herald has asked Verrall, an infectious diseases specialist before entering Parliament, about how the virus could be transmitted between two people who have never met.
The only thing we've been told about the Defence Force serviceman and the AUT student - two of six cases in the November quarantine cluster - is that they were in the same vicinity at the same time.
Interviews have yielded nothing to suggest an interaction between them, including whether they might have used the same public toilet.
Verrall, who has no ministerial responsibility for the Covid-response but is speaking from her health expertise, says there's no "smoking gun" with Covid and we only get inferences with varying degrees of certainty.
Transmission via a shared pedestrian-crossing button isn't impossible. "People do this all the time," she says as she wipes her face.
A suggestion to use long sticks to press buttons sees a light chuckle from Verrall: "Because of the number of Google searches I did related to Covid during lockdown, I started to receive Facebook ads for a device you could buy to press lift buttons."
A more reasonable way to protect us from Covid is the Swiss cheese model - enough layers of protection to prevent the virus leaking into the community, regardless of whether there are holes in any one layer.
Those layers include high levels of testing, QR codes that are displayed and scanned, secure managed isolation and quarantine (MIQ) facilities, and hygiene practices including washing hands and refraining from wiping faces.
But there have still been eight occasions since the end of July when Covid-19 has entered the community from MIQ or a port. How this happened remains a mystery for many of them.
Their frequency - about one every two weeks - have public health experts worried that another large outbreak is inevitable unless more layers are added.
"You'd have to say that's an evidence-informed opinion, given recent events," says epidemiologist Michael Baker, who calls them "border control failures".
"Based on the current pattern, unless we act, the risk is going to increase because we're seeing a rising number of infected people arriving in New Zealand - which is likely to be the case for at least the next three months."
But the Government is not alarmed because they've all been quickly contained since the August cluster, and our level 1 freedoms speak for themselves.
The important thing, Covid-19 Response Minister Chris Hipkins says, is to respond by plugging any gaps that are exposed.
Rare or unresolved Covid-19 transmission
The eight border incursions are:
• The Rydges maintenance worker, who used an MIQ lift shortly after an infected overseas arrival
• The overseas arrival who used the same rubbish bin as an infected person and was still incubating the virus when they left MIQ - six cases including three recent returnees and three household contacts
• The port worker on the same boat as infected foreign crew - four cases including the worker, two work colleagues and one household contact
• Each of the two nurses working in the quarantine wing of the Sudima Hotel in Christchurch; they have different Covid-19 genomes from each other
• The Defence Force cluster, which started via a serviceman working at the Jet Park - six cases including at least three NZDF staff, an AUT student and two close contacts.
Surface transmission is considered rare, yet the Health Ministry says a lift button in Rydges and a rubbish bin lid at the Crowne Plaza in Christchurch were two "likely" sources of infection. Masks were worn by those involved in both cases.
PPE was also worn by the Sudima nurses treating infected mariners from Russia and Ukraine, the port engineer on the same ship as infected crew who had flown in from the Philippines, and the Defence Force serviceman.
We know who they caught Covid-19 from, but not how.
The case of the serviceman is particularly puzzling, as there is no evidence of interaction between him and the Jet Park cases who are linked to his Covid genome.
The Jet Park nurse's infection is less uncertain. She was wearing PPE but dealt with a patient who removed their mask during treatment.
Other routes of transmission remain unresolved, including a casual contact of the port engineer who might have caught Covid-19 after a three-minute encounter.
If true, this would turn everything we know about close contact transmission - 15 minutes, proximity within two metres - on its head.
Unreliable information, lies and blind spots
There are two obvious explanations for mystery transmissions, says Otago University Professor and specialist public health physician Philip Hill.
The first is that our layers of protection are so good at shutting down common transmission events - droplets via close contact - that only rare ones are popping up.
The second is that droplet spread is actually the main form of transmission but we aren't seeing - or aren't being told - how it's happening.
"In health we always say 'common things occur commonly' and 'uncommon things occur uncommonly'," says Hill, who is also co-director of Otago University's Centre for International Health.
"So you have a string of transmission events occurring and it is reported that they all have uncommon explanations. That makes us uncomfortable.
"Most likely, a good proportion of those 'rare transmission events' are actually direct human liaisons that are undetected, or known and undisclosed."
Much of the information provided to health officials comes from interviews, which can't always be independently verified.
Sometimes, it's confusing. We were told the AUT student's work manager told her to go to work despite being sick, which was then denied by both of them.
And sometimes, people lie. This was recently brought into sharp focus in Adelaide, where a person was thought to have caught the virus from food packaging after buying a pizza.
Health officials thought they were dealing with an unusually contagious strain until it was revealed the man actually worked at the pizza bar alongside a security guard who also worked in MIQ.
Then there's what health officials know but aren't telling us.
A potential Covid exposure event at an Auckland spa, for example, was only revealed after US actor Wade Williams told CNN that he'd had a massage there and was later contacted by health officials.
This led to a Health Ministry statement that left some in the health sector scratching their heads. Exposure events, the ministry said, were only made public if all the contacts can't be identified.
And then there are the MIQ blind spots.
Several of the 32 MIQ facilities - including at least one where a border incursion took place - do not have CCTV coverage at all, while none of those that do have 100 per cent coverage.
There have been 76 MIQ bubble breaches since the start of August, and these don't include any that elude the watchful eyes of staff or CCTV coverage.
Hill says that's a sign of normal human behaviour and the sheer volume of MIQ traffic - up to 7000 people every fortnight.
"You'd expect, given the sheer volume of people coming through MIQ, that there would be some unauthorised mingling in MIQ."
Baker says New Zealand is in a unique position - along with other countries pursuing elimination such as China and Hong Kong - with a low number of cases so that each one can be put under the microscope.
"This means we need to be vigilant for less common modes of transmission that would go unnoticed in other countries, where there are high levels of community transmission," he said.
"A good example is concern over contaminated food that is imported chilled or frozen, and remains a potential source of the Auckland August cluster. Hong Kong and China have suggested such sources of unexplained local outbreaks."
Director general of health Ashley Bloomfield has ruled out frozen food packaging as the source of the August cluster.
Low levels of virus were found on four out of 35 swabs taken at the Americold site in Mt Wellington, but in areas where sick workers might have left traces.
One weak positive sample was detected on food packaging that was considered consistent with contamination, but ESR scientists say that surface transmission is very hard to prove.
What does this all mean for our border controls?
There is a practical aspect to case investigations, Verrall says.
"Which of these routes of transmission was the most likely? Can we patch that up? Of the less likely ones, is there any improvement we can make?"
Many changes have taken place in response to community cases in recent months, including several after the August cluster.
One of the most important was the regular testing of border-facing workers, something the Government announced in June but the Health Ministry was still rolling out in August.
Recent cases, including the Rydges worker and the Jet Park nurse, might have flown under the radar were it not for routine testing. Last week Hipkins widened the circle of workers who must be tested both weekly and fortnightly.
Others changes include:
• Mandating masks on domestic flights and public transport at level 2 after some of the August cluster was spread on a bus.
• Compulsory masks on domestic flights and public transport in Auckland at level 1 after an infectious worker in the Defence Force cluster didn't mask up on a flight.
• Zoom meetings where possible after a Defence Force worker caught Covid-19 during a meeting with the NZDF serviceman.
• Mandatory testing for foreign mariners coming into the country following the port case.
• Foot-pedal bins in MIQ following the Crowne Plaza cases.
• New guidelines for N95 masks - to be used when treating cases in their rooms, or escorting them to exercise areas or to a different facility - following the Sudima cases.
Among them was directing contact-tracers to look backwards as much as forwards; getting as close as possible to the source provides a more complete picture of the size of the spread.
And then there's the game-changing use of genomic sequencing. It was more of a tool to learn about cases retrospectively, but is now a vital part of the Government response.
The genome can show if an infection is part of an undetected outbreak, or linked to a known case. If the latter, it can even reveal how many missing links there might be between cases.
Its value was underlined again this week when it showed the Air NZ cabin crew member, who tested positive while in China, was likely infected while overseas.
Time to stop dodging bullets?
There's a roll of the Covid dice every time there's a new undetected case in the community.
The ones that do end up spreading depend chiefly on the carrier - are they a super-spreader? - and the potentially super-spreading events they might attend.
The chances of a single incursion triggering a large outbreak do not seem to be high - for the March-April outbreaks, only 19 per cent of virus introductions led to more than one additional case.
But we can only roll the dice so many times before there will be serious consequences, says Auckland University Professor Shaun Hendy, whose team has provided Covid modelling to the Government.
"We do need to tighten up MIQ or we will eventually see another large outbreak. As case numbers rise in the northern hemisphere winter, we are likely to see more infected travellers, so the risks are likely to grow."
Hendy adds that a transtasman bubble, whenever it happens, will also free up about 40 per cent of MIQ capacity for travellers from more Covid-ravaged countries.
Hill says some of the recent transmissions may not have been reasonably preventable, but it's dangerous to call any of them unavoidable.
"What you want is for anyone who is in charge of all or any part of the system to have a dogged determination to not let the virus through 'on my watch'.
"You need more than 'tick-box' infection control. You need workers themselves who know the subtleties of their work environment in great detail, applying the principles every day."
Baker is a harsher MIQ critic, saying no staff should catch Covid-19 from infected travellers, nor should any outbreaks arise from such incursions.
He and public health colleagues at Otago University - in a blogpost called "Time to stop dodging bullets?" - have been pushing for a risk-based "traffic light" border system to reduce the number of infected arrivals.
The proposal includes pre-departure quarantine and testing for those coming from high-risk countries, which wouldn't stop Covid from coming into the country, but would add another protection layer.
Baker says there also needs to be an urgent review that could also look at PPE, shared exercise areas in MIQ, and whether overseas arrivals should stay in their rooms until they have a negative day-three test.
He has also suggested a well-ventilated, purpose-built quarantine facility with no shared spaces at Ohakea airbase.
The Government has to weigh up a range of factors beyond health benefits, including economic costs, privacy issues and the impact on public goodwill.
Hipkins says that purpose-built facilities, for example, would only be worth it if there was a two-year wait for vaccines.
Imposing pre-departure rules for returning Kiwis has to be weighed up against their right to come home.
Cabinet is still considering other measures including mandatory masks at level 1 for public transport beyond Auckland, as well as various technologies to enhance contact-tracing including mandatory QR scanning in high-risk scenarios.
But it appears there are no additional measures in place to improve ventilation in MIQ hotels.
The Herald asked the Ministry of Business, Innovation and Employment, which oversees the non-health aspects of MIQ, about additional measures to improve ventilation. MBIE passed this request to the Health Ministry, which said it was constantly looking into how ventilation affects transmission, and then referred the Herald back to MBIE.
Officials are understood to have looked at gauging the success of New Zealand's MIQ, but there aren't enough similar systems overseas to give a meaningful comparison.
One place that uses MIQ is Australia, where breaches have had harsher, longer impacts.
An MIQ breach sparked the outbreak in Melbourne in June, which saw the city in lockdown for almost four months, while another breach is the cause of the current scare in South Australia, where regular testing of MIQ workers has only just started.
Hipkins says New Zealand's MIQ system as is strong as it's ever been, leaving him confident but paranoid.
The big picture is that New Zealand remains free of community transmission, with a Covid response recently rated as the world's best by the Bloomberg Covid resilience index.
Whether it stays that way remains to be seen.
Says Hill: "If we get through to the end of the pandemic without another lockdown, no matter how many little mistakes we make along the way, we'd all accept that."