New Zealand Prime Minister Jacinda Ardern has locked down Auckland for three days after its first case of community transmission for months.

Four members of a single south Auckland family have tested positive for coronavirus in the city with no known source for the infection.

Ardern said schools would be closed, people would have to work from home and certain businesses would be closed.

She said residents would be asked to return home and people who did not live in the city would be urged not to visit.

The government will offer free mass testing and pop-up clinics as it works to discover the source of the infection.

"If you are in Auckland you must work at home unless you are an essential service worker. All schools will be closed except for the children of essential service workers," she said.

She said the case came to light after one person without any history of international travel tested positive.

The person had a fever, cough and noticeable symptoms. That person's partner also had what were described as obvious symptoms.

Another six members of the person's family were tested and three were positive, including one pre-school age child.

The family were receiving treatment and going into quarantine.

"We're going to undertake widespread mass testing ... and see if we can identify those sources that would limit the need to undertake further restrictions," Ardern said.

Auckland will go to three days of Level 3 restrictions starting from noon on Wednesday.

Ardern said the government had taken the decision to "move quickly with a precautionary approach" and defended the move to put New Zealand's biggest city into lockdown.

"It would not have been possible to isolate one small part of Auckland," she said.

"The alternative is to wait - that runs the risk if we do have a wider number of individuals infected, of the spread continuing to escalate."

Ardern was asked about any parallels with Victoria's lockdown, which saw residents fined for flouting lockdown laws in droves.

"I'm not going to make an assumption of what will happen in New Zealand based on what's happened in Australia," she said.

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