This news means officials can more easily encircle the outbreak; however, experts say as the number of places of interest swells past 100 — including some with "super-spreader" potential — and because of the extra threat of the Delta variant, extensions to the lockdowns are likely.
Last night, there were 21 Delta Covid cases in Auckland, including two in hospital.
Twelve have been confirmed as part of the same Auckland cluster.
It is understood several are relatives.
A further eight are being investigated and are expected to be part of the cluster.
The other — an air crew member — is not expected to be linked because it is a border-related case.
Two people were taken to North Shore Hospital on Wednesday night.
One had worsening symptoms and the other had underlying conditions. One is in their 20s and one in their 40s.
Prime Minister Jacinda Ardern said based on genomic testing, the current cases were a close match with a recent returnee from Sydney, who arrived on a managed red zone flight on August 7.
They returned a positive result on the 9th and were transferred from the Crowne Plaza facility to the Jet Park quarantine facility that day. They were taken to Middlemore Hospital on August 16.
There was a chance additional information could emerge, but "the balance" of evidence gave enough confidence to begin searching for the missing link to the community cases, Ms Ardern said.
Middlemore Hospital was not a line of investigation at this stage due to the timeframe, but staff at the other facilities would be re-tested.
Director-general of health Dr Ashley Bloomfield said with the likely border link, their modelling suggested cases could reach about 50.
There were 362 people categorised as close contacts of positive cases, but by the end of yesterday "well over 1000" people would be contacted, he said.
The Cabinet will meet at 1pm today and announce at 3pm whether to extend the three-day lockdown, due to end at 11.59pm today for the country aside from Auckland and Coromandel, and any variations to the seven-day lockdown in those centres.
Te Punaha Matatini Covid-19 modeller Prof Shaun Hendy said the discovery of a possible link to the border was good news.
Finding the link meant there could have been fewer cases circulating in the community, but the speed with which the cluster is growing means they still estimate there could be about 100 cases.
Based on this factor and the growing list of locations of interest, a lockdown extension was likely, he said.
"It is still a fast-moving cluster and there are a lot of locations."
More than 100 have been listed so far.
Ms Ardern said essential workers, including 50,000 customer-facing supermarket workers, would now be prioritised for vaccinations.
Frontline police have also called for emergency vaccinations after it emerged just 40% of 10,000 staff across the country were vaccinated.
Ms Ardern also announced the Cabinet had agreed to make the Pfizer vaccine available for 12 to 15-year-olds, of whom there are an estimated 265,000.
It was "an important next step" and there were sufficient vaccine doses for everyone, Ms Ardern said.
Health leaders welcomed the news, GP and Maori health expert Dr Rawiri McKree Jansen saying it was particularly crucial for protecting Maori and Pasifika populations due to the younger age profile and higher vulnerability.
Dr Jansen and other Maori health leaders, including Te Ropu Whakakaupapa Uruta, have called on the Government to prioritise Maori and Pasifika of all ages in light of the recent outbreak.
