Experts say it's possible New Zealand could again escape a community outbreak - but there's also the potential for hidden transmission that would require a stricter and longer lockdown. What are the factors?
The best-case scenario
One of the best things we could possibly hear this week is that officials have a solid link to the border - which would lower the likelihood of a hidden chain of transmission.
University of Otago epidemiologist Professor Michael Baker said there was cause for optimism in that one of the three new community cases - a mother who works at LSG Sky Chefs, where she handles laundry from international flights - had a connection.
"Her workplace does have people who go air-side at the airport, which means there's a potential direct link to infected people," Baker said.
"At the moment, the only place you can get this virus is from an imported source, and there aren't many of them - so right away, you are really narrowing things down."
Under a best-case scenario, contact tracers would establish a direct link, and New Zealand's "stamp it out" playbook would quickly isolate all close contacts and any further positive cases - meaning officials would soon be confident enough to relax restrictions again.
During last year's Auckland August cluster - the country's most difficult and complex to date, with more than 150 cases involved - the city moved out of alert level 3 lockdown after three weeks, and down to level 2.5, where gatherings were capped to 10 people.
It was unclear how many days officials would need to see with no or few new cases before restrictions could be lowered - and to what level they'd be lowered to.
Covid-19 modeller Professor Michael Plank, of Te Punaha Matatini and Canterbury University, said there was a lower chance of the sort of best-case scenario New Zealand recently saw with the Northland scare, and infections within Auckland's Pullman Hotel.
"We are looking at a potentially more dangerous situation than we were there."
That was due to much more transmissible UK variant of the virus being involved - and the fact there hadn't yet been a confirmed link to the border.
"And there's obviously been more potential contacts in the community as well."
Plank said it was positive the cases didn't appear to have been to any "super-spreader" events, like large gatherings, that were responsible for many cases in New Zealand's first wave.
"Nevertheless, it's still possible for someone to infect a considerable number of people without necessarily having gone to a large gathering, simply because it doesn't have to all happen at the same time," he said.
"You might have someone who's infectious over a period of four or five days, and during that time, they infect half a dozen people."
He said another positive was that New Zealand now had excellent systems for contact tracing, testing and genome sequencing.
Even at the point of the August Auckland outbreak, contact tracing teams were exceeding the gold standard in the time taken to find an infected person's contacts - 80 per cent of contacts in 48 hours.
That meant those contacts could get into isolation and get tested, lessening their chance of spreading the disease.
Tracers would now be racing to track the progress of the virus - trying to work backward and identify the source and potentially infected people, while also trying to get ahead of the virus before it could spread further.
The worst-case scenario
The pessimistic scenario, of course, is that New Zealand has a hidden outbreak on its hands.
Plank said an important measure for modelling epidemics was the effective reproduction number, sometimes written as effective "R0", which is a measure of transmission potential for a virus.
It was the average number of people who are directly infected by a single infectious individual.
Plank said the environment under level 1 would have put that R0 measure to potentially three, given the variant involved and the lack of restrictions in place.
"There are two possible ways that the virus could have spread out into the community. One is that the cases that we know about passed it on to other people in the community.
"But because we don't have that direct link to the border yet, it could be that whoever they caught it from has also spread the virus out into the community.
"So it may be that the cases we know about caught it from someone who works at the airport or, via a chain of transmission.
"If you go back up the tree, that might have caused another branch - and there might be a whole branch of transmission we don't actually know about yet.
"Right now, we've some information about a timeline here from these three cases - but what we don't know is that there may have been cases upstream of those.
"In a bad scenario, it's possible that the virus has been spreading for two to three weeks - and this is just the first time we've picked it up."
Plank said a wider outbreak could spell the need for a harder lockdown.
"We know this variant is more transmissible - and so if it turns out that it really has established, and it's already infected a significant amount of people in the community, maybe we would need to go into a stricter lockdown to contain it."
Such was the danger of the new variant that alert level 4 would achieve similar results in containing it that alert level 3 did in containing the first strains last year.
"So what we saw in Auckland in August, on the old variants at level 3, we would actually need to go up to level 4 to get a similar result, or a similar rate of decreases in cases that we got in August."
Both Baker and Plank said which scenario New Zealand would face would be determined by what test results showed over the coming days or weeks - and, of course, whether that all-important border link could be established.
Experts have meanwhile praised the Government for shifting alert levels up.
"This new outbreak is of concern – but the Government is acting quickly and decisively by the alert level shift," said Otago University epidemiologist Professor Nick Wilson.
"The officials are spot on by emphasising the most important message: Get tested if you have symptoms or if you have been to places of interest.
"It might be that the genomic work will show a direct link with a type of border failure, but it might also be that there are a number of other cases in the community that are yet to be identified.
"Hopefully, the combination of the lockdown measures and extensive testing and contact tracing will help us get back to our valued elimination status."