It came after three members of an Auckland family tested positive, with authorities concerned about the new and active infections as there was no obvious source of transmission.
New Zealand Prime Minister Jacinda Ardern ordered the country's biggest city Auckland into a snap lockdown for the first time in nearly six months on Sunday after three coronavirus cases emerged in the community.
The Pacific island nation has been widely praised for its handling of the pandemic, with just 25 deaths in a population of five million.
But almost 2 million residents were told on Sunday to stay at home from midnight, when the level-three lockdown begins, with schools and businesses to close except for essential services.
"The main thing we are asking people in Auckland to do is stay at home to avoid any risk of spread," Ardern said.
She added that the restrictions were "just in case it could be one of the more transmissible strains of Covid that we need to act with a high degree of caution around".
It came after three members of an Auckland family tested positive, with authorities concerned about the "new and active" infections as there was no obvious source of transmission.
The city will also be ring-fenced from the rest of New Zealand, with travel in and out of the metropolis highly restricted for the next three days.
The remainder of the country will move to alert level two, Ardern said, 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," she said.
"But remember, we have been here before, that means we know how to get out of this -– together."
Too soon to judge
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.
A fourth member of the household is also showing symptoms of Covid-19 but returned a negative test.
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 at a coolstore 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.
Ardern has said New Zealand's borders were likely to remain closed for the rest of this year while the pandemic continued to rage worldwide.