Prime Minister Jacinda Ardern has once again shut down calls to close New Zealand's borders, reiterating that the Government cannot legally deny citizens the right to return home.

On Sunday, it was announced that three new cases of COVID-19 had been detected in the community. It remains unknown how the initial case - an employee at the Auckland Airport branch of LSG Sky Chefs, a flight catering facility - became infected. The other two cases are family members, with all three living in the same South Auckland household.

Genome sequencing confirmed the three people are carrying the B.1.1.7 strain of the virus - a mutated variant first detected in the United Kingdom, which is understood to be more infectious than the original strain that circulated New Zealand last year. 

Last month, leading epidemiologist Professor Michael Baker urged the Government to close the border to New Zealanders travelling from the United States or the United Kingdom. His call came amid growing concerns over the prolific spread of both the UK variant and another highly infectious mutation, B.1.351, the origin of which has been linked to South Africa. A number of Kiwis have also expressed their desire to see the borders closed on social media, with many arguing it would offer increased protection and ensure New Zealand maintains its status as a safe haven in a world stricken by the virus. 

On Monday, The AM Show host Duncan Garner echoed calls to tighten the border, pressuring the Prime Minister to justify why the Government had not scaled back the number of people permitted to enter New Zealand - or shut out returnees entirely. 

But Ardern, who has previously addressed the issue, firmly reiterated that citizens cannot legally be prevented from returning to their country of origin - leading to a tense exchange between herself and the host.

"But you can limit it?" Garner pressed.

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"Explain to me how we might do that, Duncan?" Ardern hit back.

"The fact that we have a booking system already limits the amount of risk New Zealand is exposed to as we don't have endless capacity available. Some have put pressure on us to provide more, we haven't. We believe this is a risk profile we can manage."

Garner maintained that allowing thousands of returnees to enter the country poses a significant risk to the safety of New Zealanders, with the two more virulent strains rapidly circulating overseas.

"Why don't you pull it back? There's nothing I've heard here that's convincing me at all. There's thousands that come in every month - why?" he pressed.

"Because, Duncan - you and I, and all of New Zealand, have a legal obligation to allow citizens to return to their home country. No country in the world that I'm aware of - and I've asked people to check - have barred their citizens from coming home because they legally cannot," Ardern argued.

"You cannot deem someone stateless, because it could mean they are stranded in a country they have no legal right to be [in]."

Shutting the borders would also prevent the import of critical items, including medical equipment, Ardern noted.

"Shutting the borders doesn't mean just shutting [out] people, it means you wouldn't be able to have medical equipment, imports, critical items that we need - they would not be able to arrive here without the assistance of planes, ships and crew," she said.

"We always had to have a system that was able to protect us, but the idea that we could put ourselves in a pneumatic seal and have nothing come near us is just not realistic."