The cleaner who worked on red zone planes and contracted coronavirus also cleaned green zone planes, including planes that left for Australia as the bubble opened on Monday.
The same protocols are practiced in Australia, but COVID-19 Response Minister Chris Hipkins isn't ruling out changes to the practice.
"We'll look at changing the system, if the investigation highlights that there's a need to change the system," he said.
But Otago University Professor Michael Baker says working out how a cleaner wearing personal protective equipment (PPE) was infected is the most critical aspect.
"I think there's a relatively low risk from staff members working on both types of planes, because after all, the main infection controls here are around what PPE that worker uses."
What we know is there's a clear link to a red zone passenger. The passenger touched down from Ethiopia via the United Arab Emirates on April 10 with coronavirus.
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After that person disembarked, the cleaner came onboard. Genome sequencing shows the cleaner had the UK variant - an identical match to the infected passenger.
As the air conditioning was switched off, it's possible the virus was still circulating in stagnant air on the plane.
Hipkins said he didn't want to refer to what had happened as a failure.
"There could have been a mistake somewhere in the process, which would be unfortunate," he told a press conference on Wednesday afternoon.
But Prof Baker says a systems failure is exactly the right way to describe it.
Twenty-five close contacts have been linked to the case, 17 of which are workplace contacts. The person worked three shifts while infectious with a number of colleagues.
There are three locations of interest, including the food court at the St Lukes Westfield, sparking a surge in testing nearby. Staff working at the site told Newshub they'd been relatively busy on Wednesday morning, having seen more than 100 cars in three-and-a-half hours.
"It is concerning, because I suppose the unknown always is," Auckland resident Paris Taniwha told Newshub.
"Just hope for the best and if everyone gets tested like this then we'll be fine," said another.
What isn't fine, according to Hipkins, are the fake health pamphlets being handed out by conspiracy theorists to Wellington commuters. He had some advice on what to do with them.
"They are called the rubbish bins and I would encourage you to use them - that's the only fit place for them."
He says such behaviour is dangerous - and isn't needed in the midst of a pandemic.