The Air New Zealand flight attendant at the centre of Auckland's newest border-related case was given a Covid-19 vaccination shot last week.
The Prime Minister told TVNZ's Breakfast that as a key frontline worker, the crew member got the first of two Pfizer jabs last week.
She tested positive during routine surveillance testing on Saturday.
The Herald understands the vaccination took place on March 3, three days after the flight attendant arrived home from Japan on February 28.
Last night, the Ministry of Health said that alongside a trip to a supermarket at Auckland Airport, a health appointment was currently being assessed.
All those attending the same clinic at the same time were being identified and would be contacted.
Ardern said the crew member was one of more than 9000 border, managed isolation and quarantine staff to have already been vaccinated against the virus, but the jab hadn't had time to take effect.
"This person had only just been vaccinated, so they were a priority for this exact reason," she said.
"The issue being, of course, that the vaccine takes a couple of weeks to work so at this point it wasn't quite doing its job and nor would we have expected it to, but it does demonstrate this person was indeed a priority for us."
Health authorities are now working on identifying the strain's genome sequence.
It is not clear whether the nurse who gave the flight attendant her vaccine is a close contact.
Neither is it clear if every subsequent person who that nurse vaccinated has also been contacted about a possible infection being transferred from the vaccinator.
The Health Ministry said specific advice would be provided to individuals identified as contacts by public health staff about the steps they needed to take.
Air New Zealand and the Ministry of Health have been approached for comment.
Yesterday, the flight attendant was moved to Auckland's quarantine facility but not before their partner - a close contact - enjoyed a round of golf with friends at Remuera Golf course enjoying the region's newly-relaxed alert level settings.
He and two others in the household have been tested and their results have returned negative.
The course is undergoing a deep clean today as a precaution, general manager Chris Davies confirmed.
Yesterday, 14 other aircrew on the same international flight as the latest case were in the process of being contacted, isolated and retested.
This morning, Ardern said health officials were looking into whether there needed to be more restrictions on international crew, depending on what location they're coming from.
"As circumstances change in different places [countries], we look at whether or not we need to be heightening what we're doing," she told Breakfast.
She said in this latest case the flight crew had been tested on February 22 and 28 and again on March 6.
"They obviously wear PPE as part of their job, if they are requiring accommodation in the site they were staying they're not allowed to leave, they're not allowed to have contact with any other people," Arden said.
"So they do have quite tight protocols around them."
University of Auckland vaccinologist Helen Petousis-Harris said while vaccinators wore masks, it wasn't necessarily normal practise to wear gloves when vaccinating.
"We're also not vaccinating in a situation where we've got lots of the virus around as such," she said.
But getting the Covid-19 vaccine wasn't like "waving a wand and you're instantly protected".
She said it took a couple of weeks to start getting protection and the worker wouldn't be considered immune, as the time that had lapsed was very short since they had received their own shot.