Auckland faces two scenarios - one where its new Covid-19 case is linked to the infection New Zealand Defence Force cluster and the breach is contained, and another where we're simply looking at the tip of the iceberg. Science reporter Jamie Morton asked experts about each one.
The hopeful scenario
The best-case scenario is that scientists genomically link Auckland's mystery case - a student in her 20s - to the Defence Force cluster.
So far there's good reason to hope there is a match - Health Minister Chris Hipkins says there's a strong likelihood, with more to be revealed at 2pm - and that could spare another lockdown.
"The source of the infection is the crucial question," said Canterbury University mathematician and Te Punaha Matatini modeller Professor Michael Plank.
"Because if genomic sequencing shows up a match with a case that's already on the record, then that will give us a pretty strong line of inquiry."
Plank said the other advantage New Zealand had now, as opposed to the time of the first outbreak, was a strong and effective contact tracing system.
"We've become very good at tracing down everyone's contacts and getting hold of them pretty quickly - it's also improved since the August outbreak."
Even at that point, contact tracing teams were exceeding the gold standard in the time taken 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.
Plank said tracers would be carrying out both forward and backward contact tracing.
"Normally, when we talk about contact tracing, we're talking about forward contact tracing, which is you're trying to get ahead of the virus and stop it from continuing to transmit, but the backward contact tracing in a situation like this is really important as well, and that's about trying to find the source of infection.
"Until you know, you can't sort of trace back any further, until you hopefully you find a link to the border.
"But tracers will have been going through all this person's close contacts, and identifying quickly whether any of them were working in managed isolation and quarantine facilities, or were border-facing workers. That would be one possibility.
"But if this link doesn't show up in their close contacts, then it will remain a bit of a mystery for a while longer, potentially."
Plank said if the infection was found to be a secondary case, rather than the latest in a hidden chain, we could be more optimistic.
"That would mean there's definitely potential that we can contain it."
It was also good news that the case appeared to have had relatively limited movement.
The woman lived alone and worked in a customer-facing role in their job at A-Z Collections on Auckland's High St.
She called in sick to work after being told to isolate but after a conversation with her manager went to work and wore a mask.
The woman's three close contacts are colleagues and friends and are being moved to a isolation hotel. They are all reported to be feeling well.
Her other visits were at Smith and Caughey's on Queen St, Red Pig Restaurant, Starbucks Queen St, Sunnytown Restaurant, The Gateau House on Queen St.
The woman also took a number of "very short" Uber trips to work - drivers are being contacted and asked to isolate - and made a brief visit to AUT's City Campus Student Hub.
"If we look at this person and see they've not had a large number of contacts during their infectious period, then maybe they've infected only one or two other people.
"But if we're, if we get unlucky, maybe they've passed it on to someone else maybe in a crowded bar or in a restaurant.
"And then that could potentially cause quite a large number of cases quite quickly."
The pessimistic scenario
The pessimistic scenario - obviously besides the breach behind the case leading to a wider and uncontrolled outbreak - was Auckland or other areas having to go back into lockdown.
Yesterday, Hipkins said it appeared the case was currently contained and that there was no danger of an "enhanced risk" outside the CBD.
Alert level 3 applied when there was a "high risk" the virus was not contained, and there were "multiple cases of community transmission occurring".
Auckland spent nearly three weeks under alert level 3 in August - but the scale of infection then, when four cases of community transmission were initially discovered, was much wider than it appeared to be now.
Plank said one other concern - and aside from the fact the infection source still remained unknown - was that the woman lived in the Vincent St Residences apartment.
"We've seen situations in other countries where they can spread very rapidly through an apartment building, because they have a lot of shared areas," he said.
"So that's definitely a cause for concern. Hopefully, they'll be getting everyone in that apartment building tested as quickly as possible."
Another worry was the open environment in which the case might have been able to spread, with no restrictions in place, and in a busy area.
Many people typically moved within downtown Auckland - there are 100,000 workers in the CBD currently being told to keep their movements and contact to a minimum.
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 that are directly infected by a single infectious individual.
Plank said the environment under level 1 would have put that R0 measure above one.
"It was probably close to between where we were in March or April, which was around 1.7, and the time before the August cluster, which was about two and a half," Te Punaha Matatini director and modeller Professor Shaun Hendy said.
"The crucial thing is whether there's a genomic link found to the Defence Force cluster - and that they haven't ruled it out is encouraging.
"We should know later today whether there's an exact match, and that tells us this case is probably just a secondary case from that [Defence Force] cluster.
"That's still not great, but if we can find the direct link, we're unlikely to be back in the August situation.
"Our guess is that the August outbreak might have been going on for a couple of weeks before we found it, whereas if we find a direct link, we're likely to have caught it within the first week.
"Both our testing rates and use of the app are higher than they were in early August, so I think we are likely in a better position now than we were then.
"Nonetheless, it is important that we all remain vigilant, especially those in the Auckland region."