TWO new coronavirus infections which sent New Zealand's biggest city into lockdown Monday are due to the UK variant of the disease, the first to be detected in the country, officials said.

The Health Ministry said genomic sequencing identified the infections in Auckland as caused by the highly-contagious strain first found in the UK and said there was no link to any other positive cases detected so far in New Zealand.

"This result reinforces the decision to take swift and robust action around the latest cases to detect and stamp out the possibility of any further transmission," the ministry said.

There were no results yet from sequencing of a third case linked to the Auckland cluster, all members of the same family, which emerged Sunday and prompted Prime Minister Jacinda Ardern to order a three-day lockdown in Auckland.

It was the first clampdown in nearly six months in the Pacific island nation, which has been widely praised for its handling of the pandemic, with just 25 deaths in a population of five million.

Almost two million Auckland residents were told on Sunday to stay at home from midnight, when the level-three lockdown began, with schools and non-essential businesses forced to close.

The city has also been ring-fenced from the rest of New Zealand, with travel in and out of the metropolis highly restricted for the next three days.

- Tracing source of infections -

The remainder of the country was placed on alert level two, with people required to wear masks on public transport and gatherings limited to a maximum of 100 people.

"I know we all feel the same way when this happens -- not again," Ardern said in announcing the measures Sunday afternoon.

"But remember, we have been here before, that means we know how to get out of this -- together."

Health officials said a mother -- who works for a company providing laundry and catering services to international flights -- and her daughter tested positive Saturday, before the father returned a positive sample Sunday.

New Zealand director-general of health Ashley Bloomfield said the initial focus was on the mother's workplace "because of its obvious connections to the border".

But he added it was "too soon to rule in or out" any source of transmission and the woman had not been at work for eight days before testing positive.

Auckland spent more than two weeks in lockdown last August after a virus outbreak was linked to a worker handling imported frozen freight, but New Zealand has largely been enjoying relaxed restrictions for months.

Several cases of Covid-19 were also detected in the city three weeks ago, before being traced back to a hotel where the people had completed quarantine after arriving from overseas.

Ardern has been widely praised for her management of the pandemic, with New Zealand recording fewer than 2,000 infections.

The country closed its borders and implemented a strict five-week lockdown in March and April last year, with occasional virus outbreaks since then being quickly contained.