The source of the latest case of Covid-19 at New Zealand's border has been discovered.

Covid-19 Response Minister Chris Hipkins told Newshub genome sequencing results overnight show the case is linked to a passenger who arrived in the country on April 10.

He says it suggests there are no intermediaries in the case, and work is now under way to connect the cleaner with the plane the passenger arrived on.

Hipkins said all test results from close contacts and household members have so far come back negative.

Chris Hipkins told RNZ he was awaiting confirmation that the worker cleaned the plane of the most closely linked returnee.

"The match is exactly the same, it's identical, so that would suggest it's person to person [contact]," he said.

An investigation would uncover how the person picked up the virus, he said.

It was possible that with the air conditioning switched off in planes as they were cleaned, droplets from someone with Covid-19 positive could still be in the air, he said.

"The nature of a plane if the air conditioning is not going is that the air is quite stagnant. It's possible that that was still hanging in the air when someone went in to clean the plane, particularly if that happened quickly after it arrived."

Two vital questions hang over the case of the fully vaccinated border worker who has tested positive for Covid-19, a leading epidemiologist says.

The fresh case - which came on day two of the long-awaited transtasman bubble - is the second airport-based cleaner to be infected through their work.

The new case worked cleaning planes from high-risk countries and had received both vaccine jabs.

The earlier case was an LSG Sky Chefs worker who is believed to be the index case of the Valentine's Day Papatoetoe cluster earlier in the year which infected 15 people.

Three new locations of interest have been identified relating to the latest Auckland Airport worker case.

They are:

• Westfield St Luke's Food Court, Saturday April 17, 12.15pm to 2.30pm
• Bunnings New Lynn, Saturday April 17, 2.30pm to 3.50pm
• Movenpick Dominion Rd, Saturday April 17, 5.15pm to 7.20pm

"The person is currently isolating at home while they are being interviewed by health officials and following this they will be transferred to the Auckland quarantine facility," the Ministry of Health said in a statement.

"Five household contacts have been tested and have returned negative results. Close contacts from the person's workplace are being identified, isolated, and tested.

"So far, we have identified that the person has 16 close contacts. This number will likely change as further scoping of this person's movements identifies other people they have been in close contact with."

The person's work at Auckland Airport is their sole employment, and their role is non-public facing.

Additional pop-up testing had been set up at Auckland Airport on Tuesday in addition to the testing site already running at the airport.

Both the Australian and New Zealand governments stressed yesterday that new cases of the virus were expected and the latest infection did not threaten the transtasman travel bubble.

Epidemiologist Professor Michael Baker said two huge questions hung over the new Covid-19 case: how the person became infected - given that they had been fully vaccinated - and what the mode of infection was.

"This case is very interesting in a lot of respects. Firstly, the fact that they were only found on routine testing, they didn't have symptoms, they were vaccinated, it's quite likely that they may have had a very low ... risk of infecting other people in those circumstances.

"This is right at the core of one of the great questions of vaccination, does it prevent you from transmitting the virus?

"Obviously, in this person's case, it didn't stop them from getting infected. It might be that their viral load remained very low, and their risk of infecting other people very small. That's what we're hoping the vaccine does."

The infected person works as a cleaner at Auckland International Airport, cleaning planes that have flown internationally from countries where Covid-19 is widespread.

The LSG Sky Chefs employee from February worked in a laundry facility, dealing with linen and other materials coming off planes.

Baker said the new positive case raised the possibility of the virus being transmitted on a surface or aerosols.

"Aerosols can linger in the air for some time after an infected person has left an area.

"I don't think that has ever been documented, that someone has ever been infected from an aerosol after passengers have disembarked [the aircraft]. In the end, it's quite hard to distinguish aerosols from surfaces because of course both of them could be leftover from an infected person. I guess it just remains to be seen."

He said genome sequencing was critical as it would confirm if the new case's infection matched any other known genomes that had come through the border at that time.

"We've already had examples of two significant outbreaks where we never found the cause - it may well be this will be another one."

Confirmation of the positive result came just a day after the transtasman bubble with Australia opened, allowing quarantine-free travel between the two countries.

But Baker said he didn't think the case threatened those arrangements.

"I don't think this case represents a threat to the green zone, because this is an example where it is a form of border failure, but actually at the very low end, and it has been detected very quickly. So this is an example, as I think the Government will say, of the system working well, of routine testing."

E Tū union head of aviation Savage told RNZ it was important the public not put blame on the airport worker.

"In situations like this a lot of pressure gets put on aviation workers and airport workers ... sometimes there's a lot of blame put on to them.

"And as we know this virus is very difficult to contain, and I think the public should be reassured that there are very good health and safety regulations in place."

It was unfortunate the highly transmissible virus could sometimes slip through even strict protocols, Savage said.

The worker had been tested weekly for Covid-19 as part of routine surveillance testing, said the Ministry of Health.

They were tested on Monday at their workplace and that test returned a positive result yesterday. Their previous tests were all negative.

The person was currently isolating at home while being interviewed by health officials but would be transferred to the Auckland quarantine facility.

The ministry said that based on current evidence, the risk to the public appeared low.

The three locations of interest identified so far are:

• Westfield St Luke's Food Court, Saturday April 17, 12.15pm to 2.30pm
• Bunnings New Lynn, Saturday April 17, 2.30pm to 3.50pm
• Movenpick Dominion Rd, Saturday April 17, 5.15pm to 7.20pm

Anyone who had visited the three locations of interest during the relevant times are considered casual contacts, and should monitor their symptoms for 14 days, stay home if they develop symptoms, contact Healthline on 0800 358 5453 and get a test.

Asked about whether the new case threatened the travel bubble, Prime Minister Jacinda Ardern said she had always said there would be cases - "Australia accepts that".

The vaccine was 95 per cent effective. That meant people could still get Covid, but the health effects wouldn't be as bad.

"It is working as intended ... it's doing it's job," she said of the vaccine.