A border worker at Auckland Airport who was reported positive for COVID-19 on Tuesday appears to have picked it up from a passenger who arrived earlier this month.
COVID-19 Response Minister Chris Hipkins said on Wednesday morning the cases had been genomically linked.
"Good news overnight," Hipkins told The AM Show. "We did get the genome sequencing back last night. That shows us there is a link to a passenger who arrived in the country on the 10th of April so that timing kind of works...
"It would suggest there are not potentially other intermediaries out there as well, so that's helpful for us... It's a very hopeful sign for us."
The worker, who cleans planes, and their close contacts have been isolated. None of them have tested positive yet.
"It means we're in this early and should be able to contact-trace our way out of it and contain the case," said Hipkins, playing down the chance the newly-opened travel bubble with Australia would have to be closed.
This is the fourth case recorded since July last year outside of managed isolation and quarantine facilities (MIQ) that could be deemed a border breach. Including MIQ, there have been 15.
"This is way too high. We should be tightening our systems to really reduce border failures to make them really rare events," University of Otago epidemiologist Professor Nick Wilson told Newshub on Tuesday afternoon. "It is concerning that these cases keep happening and they are really putting the success of the trans-Tasman bubble at risk."
But Hipkins said it was unfair to characterise it that way.
"When he uses language like 'border failure' in a case like this, it's effectively a criticism of the person concerned. This person has followed all the rules and done all the right things - ultimately we're dealing with a virus."
Hipkins said he's spoken to Dr Wilson, and said the professor's answer was to "shut the border to everything".
"I don't agree with him on that, it's not practical. We couldn't stop everything else coming in - for starters, we'd stop all our medical supplies coming in and that would have a catastrophic effect."
Hipkins said as long as COVID-19 is "rampant" around the world, there would be "no such thing as a 100 percent foolproof system".
"We can make the system as tight as we possibly can, but there will on occasion be something that will happen."
As long as the chain of transmission is known, authorities have previously said there is no need for a lockdown.
Experts have also raised concerns passengers from Australia, the Cook Islands and Niue - who come through the airport's 'green zone' and don't require a stay in MIQ - are being processed by staff who also work in the 'red zone', where passengers from elsewhere in the world are processed.
"On the face of it, it just doesn't seem good to have any mixing of the red zone and green zone workers," Dr Wilson told Stuff. "They're meant to be entirely separate."
Hipkins said it was no different than letting red zone workers go home each night.
"The people working at the border, if you're working in the red zone... you take all the appropriate infection prevention and control measures. You then go home at the end of the day and your movements are not restricted - you can still go to the supermarket, you can still go elsewhere."
The same measures used at the end of the day are used when switching between the red and green zones, Hipkins said, including full changes of personal protective equipment.
"There are careful infection prevention and control measures here."
The Ministry of Health said much the same to Stuff.
"From a public health perspective, provided the stringent red zone precautions for workers are followed while in red zone areas, there is no reason why they couldn't also work in a green zone area."