An outbreak source investigation centred on Auckland's Crowne Plaza MIQ has so far failed to discover how the virus spread from the suspected "primary case" into the community, as officials try to track down two people.
Health officials are looking at an atrium thoroughfare inside the Crowne Plaza building as it was used by six people at the time the earliest-identified case in the outbreak was in the nearby hotel lobby.
Of those six, four had been tested and three had so far returned negative tests. The fourth person is in the process of getting one.
"There are two people still to be identified, which police are assisting with," the Ministry of Health said today.
While the space between the MIQ and the thoroughfare was divided off, the ministry said there was still the possibility of air flow between the two spaces.
"It has been confirmed the case was indoors while a very small number of people walked in the open walkway, which is well ventilated."
The ministry clarified that the thoroughfare was not the outside walkway used to access the nearby Huawei Centre.
So far, the source investigation has focused on the Crowne Plaza, where the infected traveller arrived on August 7, and the Jet Park Hotel quarantine facility, where they were transferred to after testing positive on August 8.
Yesterday, Covid-19 Response Minister Chris Hipkins said transmission from border staff into the community had been "almost ruled out as a possibility", as more than 400 staff across both facilities had returned negative tests.
While the "primary case" was also transferred to Middlemore Hospital on August 16, this site had been essentially excluded from the inquiry given the case entered care just a day before lockdown, and days after other cases in the outbreak became symptomatic.
The primary case has also been genomically linked to three other positive cases who were in a family bubble staying in the room next door.
As a result of the link between those cases, officials quickly put in place post-departure day five testing for returnees who were in the Crowne Plaza, and who were on the same floor and whose stay over-lapped with the infectious period of that original case.
Otago University epidemiologist Professor Michael Baker said it remained to be seen whether the family bubble in the hotel - who tested positive on their day 12 test - had been able to pass on the virus somehow themselves.
Specifically, he questioned whether the family had used the hotel's exercise area after becoming infected.
"The [primary] case must have been highly infectious when they were in that two-day period between arriving at the hotel and being transferred to Jet Park," he said.
"We also know the incubation period can be as short as four days with Delta - so in effect you could have other people becoming infectious after a very short period."
Prime Minister Jacinda Ardern said today that the infected traveller had not visited the exercise area themselves, which left the atrium area as the focus point.
"We are keeping open the lines of investigation at this time."
That included looking at all movements and footage, and staff.
"But there is nothing to report at this point."
Asked why the Crowne Plaza was still open, Ardern said it was not yet established what the issue was, but extra precautions were being looked at while it was investigated.
Baker said many people would also be wondering how many links there were in the infection chain between the primary case arriving in the country on August 7, and the confirmation of the virus in the "index case" or "Case A" - a 58-year-old Devonport man - 10 days later.
While the two cases had been genomically matched, in large outbreaks, scientists could find multiple identical virus genomes without being able to infer the direction of transmission.
Media were also told today that officials had detected cases that would have pre-dated Case A's infection, which itself was a relatively early case in the wider outbreak.
Considering what we understood about the outbreak so far, Covid-19 modeller Professor Shaun Hendy expected there were likely two steps between the primary and index cases.
Would it matter if we found these missing chains?
"Overall, probably not - but if we able to absolutely nail it down, it might give us an indication of another chain that's out there that hasn't been detected yet," he said.
"But that's increasingly unlikely now, given the amount of surveillance that's been done - and it becomes less important the longer we spend in alert level 4."
Baker and Hendy agreed the real value in uncovering the full picture of how the virus spread was in improving the system to ensure it didn't happen again.
Concerns had already been raised about the potential for the virus to leak out from the Crowne Plaza into nearby access spaces. One person told Newsroom last week it was an "outbreak waiting to happen".
Baker added the outbreak should prompt a rethink about how New Zealand managed the Covid-19 risk.
"The big takeaway for me is that, if we're going to get between now and the end of the year - a critical period where we're ramping up vaccination - we really have to look at how we can reduce the number of infected people arriving into New Zealand."