Jacinda Ardern says the sheer number of possible exposure events has prompted Auckland into a second lockdown in a fortnight.
On Sunday, New Zealand's prime minister defended her decision to put the country's biggest city into a week-long snap lockdown.
Ms Ardern called a 9pm press conference on Saturday announcing the return of restrictions for the second time in this month.
Auckland will be locked down at alert level three for the next week, while the rest of NZ moves to alert level two - which caps gathering sizes.
As a result, two touring Australian sides - the men's T20 cricketers and the Diamonds - will play their series with NZ behind closed doors this week.
Both lockdowns have been caused by a single cluster, first identified in the South Auckland suburb of Papatoetoe on Valentine's Day.
Ms Ardern's government was confident enough to lessen restrictions on February 17 after a three-day lockdown on the basis that large swathes of the community had been asked to self-isolate.
Since then, a small amount of community cases have emerged, without a return to lockdown.
What was different about Saturday's new case - a student at Manukau Institute of Technology - was that officials could not find a link to previous cases.
The absence of a link suggests further undetected spread in the community.
However, by Sunday morning, health officials used genomic sequencing to link the new case to the Valentine's Day cluster.
Why then was Auckland still heading into lockdown?
"The number of exposure events for the latest case with potentially large groups of people over a sustained period of time," Ms Ardern said.
"That high number of events means it's more. It's more than likely, there will be additional cases in the community but they might not show up in test results for a few more days yet."
On Sunday, Director General of Health Ashley Bloomfield announced one new case, a household contact of other cases who was already in quarantine.
The new lockdown has triggered a range of emotions, including anger at two of the recent cases for not following orders to isolate.
One of last week's new cases worked a shift at an Auckland KFC while waiting for a test result, which came back positive.
Saturday's case also went to school, to the gym and to various shops when they should have been at home.
COVID-19 Minister Chris Hipkins said the latest case made "some big mistakes".
"This potentially was avoidable if everybody had done exactly what they are asked to do," he said.
Ms Ardern also apologised to organisers of Auckland's Pride Party after they were incorrectly advised by officials to shut down Saturday night's event before the lockdown kicked in.
"I really appreciate the fact their pride month has been deeply affect by these events and I'm really sorry that's happened," she said.
Australian Associated Press