Several possibilities could account for the source of the latest community outbreak, Covid-19 data modelling expert Shaun Hendy says.
Three new community cases were announced by Covid-19 Response Minister Chris Hipkins and Director-General of Health Dr Ashley Bloomfield yesterday, prompting an increase in alert levels across New Zealand.
The cases have been identified as a UK variant of the virus which is more transmissible, and there's no clear link to the border yet.
The three cases - a mother, father and daughter - are all in quarantine. The mother works for laundry and catering company LSG Skychefs, which services planes at the airport.
Read more on the latest Covid-19 outbreak:
- Follow RNZ's live blog for the latest Covid-19 updates
- View updated locations of interest here
- Alert Level 2 and Level 3: What they mean
Dr Hendy, a professor at Auckland University, told RNZ's Afternoons that like the August outbreak, the source of infection may never be known - but there were several possibilities.
One possibility that had been ruled out by genome testing was any connection to recent travellers who had tested positive in New Zealand.
"Staff at the ... company will be getting tested at the moment and that will help us determine if that's the route of transmission for this family," he said.
"Other possibilities are air crew ... and then they've left the country on another flight ... or someone who's been in transit, so there have been a number of people who have come in and then transit to Australia and we don't pick up their genomics."
If the case was connected to Australia there was a chance of matching the genomic sequencing to Australia's, which did detailed testing like New Zealand, he said.
One of the new cases is a student at Papatoetoe High School, and Dr Hendy said test results from the school and her mother's workplace would provide more information.
If cases were found at the school it could instead prove to be the original source of transmission, he said, noting that was purely speculative.
"We are not sure about the order in which the family has become infected, that'll be one of the things that the contact tracers will be really trying to get to the bottom of. We're assuming that it was the mother who was most likely the index case but you'll be wanting to rule out that it wasn't the daughter."
The government's decision to move to alert level 3 in Auckland was "prudent", Hendy said.
The move out of alert level 3 in Auckland would depend on what information is gathered over the next few days.
If a large cluster like the August one eventuated, then a longer lockdown would be required, and people should be prepared for a lockdown of several weeks.
On the other hand, a clear link to the border and no further cases could mean alert levels dropping again, he said.
"If we'd learnt about a link to another MIQ case this morning, then we'd probably be thinking about, or starting to think about moving out on Thursday. Now we need more information and it will really come down to the information that can be gathered in the next couple of days."
Two of the cases also visited the Taranaki region, but officials did not increase to alert level 3 there as it was unlikely they were infectious when they visited.
"Level two's probably the right thing to be using in Taranaki," he said.
Officials advised people in Taranaki who visited the same places as the pair during their trip to get tested "out of an abundance of caution", but people who were not in those places and were not symptomatic need not get tested.
Hendy said New Plymouth was unlikely to be the source of transmission and was "downstream" from the cases.
New Zealand's systems were good at handling downstream cases, Hendy said, so he believed keeping Taranaki in alert level 2 was the right approach for now.