New Zealand's dream run of 102 days without locally transmitted coronavirus ended Tuesday, prompting a stay-at-home lockdown order for the country's largest city.

After receiving global praise for successfully containing the virus, Prime Minister Jacinda Ardern said four cases had been detected in a single family in Auckland from an unknown source.

"After 102 days, we have our first cases of COVID-19 outside of managed isolation or quarantine facilities... we have also planned and prepared for it," Ardern said.

As a result of the infections, mobile phones nationwide received an emergency text that set off blaring alerts and said "if you are in Auckland STAY HOME... and SAVE LIVES".

Until Tuesday, the World Health Organization had hailed New Zealand as an example to others for having "successfully eliminated community transmission".

It has reported just 22 deaths in a population of five million and had previously held off community transmission since May 1.

Kiwis had enjoyed a near-normal lifestyle with no social distancing and spectators allowed at sports and cultural events, although borders were strictly controlled.

But health authorities had repeatedly warned people not to be complacent and said a second wave of infections was "inevitable".

Ardern said she understood the disappointment of New Zealanders who believed the virus had been quashed after a strict seven-week lockdown earlier this year.

"It was perhaps easy to feel New Zealand was out of the woods, my request is not to feel dispirited or disheartened," she said.

"Of all the countries in the world, New Zealand has gone the longest without a resurgence -- but because of that we always knew we had to plan, and we've done that."

'Tricky' virus

Auckland will be locked down for at least three days from Wednesday and some social distancing restrictions will be reintroduced in the rest of the country.

Health director-general Ashley Bloomfield was confident the measures would again contain the spread of a "tricky" disease that had thwarted authorities elsewhere.

"We cannot afford to let this virus spread," he said.

Ardern acknowledged her announcement had prompted panic buying at New Zealand supermarkets but urged shoppers to calm down.

"There's no need, they're an essential service," she said.

Ardern downplayed any impact on a national election scheduled to be held on September 19, although she said campaigning over the next few days would not take place in Auckland.

Leader of the centre-right National Party, Judith Collins, said she had suspended all campaigning engagements for Wednesday and had questions for the government.

"This will come as a shock to all New Zealanders... (we) will be seeking an explanation and clear answers about the situation we now find ourselves in," she said.