A man is self-isolating in Northland after travelling to New Zealand from Perth, despite the city being in lockdown.
The breach was found during routine passenger checks after the man arrived in Auckland on Monday.
The people of Northland have questions, the main one being: Where is he currently self-isolating?
Whangārei Mayor Sheryl Mai said Northland residents could not fathom how it happened.
"It's just unbelievable."
She told RNZ's First Up that the previous Northland case had also created fear in the community.
"There's a sense of disbelief and many questions: How did this happen, where is this person? Northland is a large region, are they next door, are they miles away? Did they shop on their way up from Auckland, have they been exposed?
"Just many questions and all of them are unanswered."
Mai said even with the best of plans for the trans-Tasman travel bubble, this incident - where the man deliberately changed flights - showed "somebody could thwart the system" and there was a "loophole" that could be exploited.
"People are people and they can be sneaky."
Travel bubble relies on people's 'honesty, integrity' - INZ
Immigration New Zealand manager Peter Elms said it was not a loophole.
"Quarantine-free travel does rely on people ... to do the right thing and comply with the rules."
He told RNZ's Morning Report that New Zealand would always be vulnerable to individuals who broke the rules.
"In this case what happened was that the individual had a ticket booked from Perth to Sydney and then onwards to Auckland. At some stage, that ticket from Sydney to Auckland was cancelled and subsequently, he booked a separate ticket with a different airline on another flight from Sydney to Auckland.
"By the time border agencies had matched the two itineraries with the individual, he'd arrived in Auckland and we had to respond after his arrival."
He said there were protocols in place to ask passengers before they board the plane if they had been in contact with a Covid-19 patient. But in this case, it seemed that the question was not answered correctly, or maybe the question was not asked at all.
There were defences in place.
"The first safeguard ... it does rely on people's honesty and integrity."
People needed to heed instructions, Elms said.
"The second safeguard is that we have airline who check people's eligibility for travel and ask questions on behalf of the government about people's circumstances."
He said airlines could ask for evidence or ask passengers to sign a declaration but it came down to a degree of honesty.
"And the third safeguard is we've got border agencies who undertake passenger screening, who will be able to identify some travellers who may well be ineligible because of history in the country."
The individual was yet being questioned by authorities, and their answers would determine prosecution, Elms said.
"Let's let the investigation take its course."
Risk 'incredibly small' - epidemiologist
Melbourne University epidemiologist Tony Blakely said the data systems between Australian states were not the best and it was "hard to stop this state hopping".
He told RNZ's Morning Report there were a few cases where people have flown to another state before heading to their destination state of choice.
It was a matter of honesty, he said.
"I don't think it's a major threat just here."
There was a need to put in extra checks and messaging, he said.
"The risk here is quite small.
"Putting it in perspective, given that the infection rate in WA (Western Australia) and anywhere in Australasia at the moment is so low and the pre-emptive lockdowns are being taken to stop the spreading ... the risk is incredibly small."
Nevertheless, it was important to plug the gaps, he said, adding that it would be smart to make an example of this and have some repercussions.
Quarantine-free travel between New Zealand and Western Australia resumed from midday today.