Ms Ardern marked her first Monday back from summer holidays by getting a third shot of Pfizer vaccine in Auckland.
The booster is seen as crucial for resistance against the Omicron variant currently ravaging many countries, including Australia, but is yet to take hold across the Tasman.
"We know that with Omicron it's a case of when, not if," Ms Ardern warned.
New Zealanders must wait four months from their second shot to receive a booster.
Until the weekend, Kiwi officials had picked up just three community cases of the Omicron variant.
The first was a British DJ who broke isolation rules to party in Auckland with the virus in late December, quickly followed by Air New Zealand crew member and one of their household contacts.
On Saturday, routine border testing identified a worker with COVID-19, and on Sunday, genomic testing confirmed the woman was the fourth Omicron case.
Ms Ardern said contact tracing was yet to produce evidence of community transmission.
"All of the testing we have undertaken around the (border) worker has come back negative," she said.
"It is very encouraging to see the close contacts and in particular household members have returned negative tests so far."
Further close contacts from a bus trip are yet to be tested, but Ms Ardern noted the woman with COVID-19 "did everything right" and was wearing a mask on her journey.
On Monday, the government also removed the requirement for unvaccinated Aucklanders to produce a negative test before leaving their city.
This was a key measure designed to stop the spread of the virus outside New Zealand's biggest city, the centre of the Delta outbreak which began in August.
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