New Zealand's largest city is being placed in a three-day lockdown following the discovery of three unexplained coronavirus cases in the community.

Prime Minister Jacinda Ardern announced the move for Auckland after an urgent meeting with other top politicians in the cabinet.

Ms Ardern had planned to attend the Big Gay Out on Sunday, an Auckland festival that attracts tens of thousands of people. She ended up cancelling those plans and returning to wellington to manage the outbreak.

"I know we all feel the same way when this happens. We all get that sense of 'not again'. But remember, we have been here before and that means we know how to get out of this again, and that is together," said Ms Ardern.

The rest of New Zealand will also be placed under heightened restrictions although will not go into lockdown, Ms Ardern said.

The lockdown is the first in New Zealand in six months and represents a significant setback in the nation's largely successful efforts to control the virus.

New Zealand had successfully stamped out community spread, and many people elsewhere in the world looked on in envy as New Zealanders went back to work and began attending concerts and sporting events.

The greatest vulnerability for New Zealand has been at the border, where new cases are regularly caught among returning travellers.

Despite precautions, there have been several times when the virus has leaked out from the border before being controlled again.

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New Zealand’s Covid-19 response minister Chris Hipkins said: "We are gathering all of the facts as quickly as we can, and the system that served us so well in the past is really gearing up to do so again."

Mr Hipkins continued: "New Zealand has kept Covid-19 contained better than almost any other country," but added that as they have said before, there is no such thing as “no risk”.

In the latest case, an Auckland mother, father and daughter caught the disease. Officials said the mother works at a catering company that does laundry for airlines, and officials are investigating whether there is a link to infected passengers. Officials said the woman had not been going aboard the planes herself.

New Zealand, with a population of five million, has reported a total of just more than 2,300 cases and 25 deaths since the pandemic started.