A 12-day extension of Auckland’s Covid-19 restrictions has been announced by New Zealand’s PM Jacinda Ardern, after a cluster of cases grew to 29.

Four people tested positive on August 11 in Auckland. A three-day lockdown was brought in the following day, now extended by 12 days.

There are four “alert levels” in the country, and Auckland has been on Level 3 since August 12.

The rest of the country is on Level 2, and PM Ardern said both would be extended.

New Zealand has had success containing coronavirus, and went 102 days without a community transmission.

The origin of the cluster in Auckland – New Zealand’s largest city with a population of 1.5 million – is still being investigated.

Jacinda Ardern said the decision to extend the restrictions was “in keeping with our cautionary approach and New Zealand’s philosophy of going hard and going early”.

She said that, in 12 days’ time, she thought “the cluster will be identified, will be isolated, and we can move to Level 2 in Auckland with confidence”.

All 29 cases “remain linked to one cluster centered in Auckland”, PM Ardern said, adding that 38 people are in government quarantine.

However, she said that, although the first cases of the new outbreak were confirmed in Auckland on August 11, contact-tracing had uncovered an earlier case, involving a shop worker in the Mt Wellington district of Auckland who became sick on July 31.

“In terms of the ongoing investigation to identify where the virus originated from, there are still no clear connections at this point,” she said.

However, the prime minister said genomic testing and contact-tracing suggested the current outbreak was not linked to border entry points or New Zealand’s previous outbreak in March.

New Zealand could expect to see more cases from the cluster, PM Ardern said, adding: “It will grow before it slows.

“And it may continue to be linked to schools, churches and social gatherings, as it has done to date. We also know, based on overseas experience and our own, that it is possible to contain a cluster or outbreak without ever being able to identify its origin.”

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Under Level 3, which is now in place in Auckland, all public places – such as libraries, museums, cinemas, food courts, and playgrounds – must be closed.

People must work from home where possible, and children are encouraged to “learn from home”. Residents have been told to stay in their “household bubbles” when not at work or school if they have to attend.

Supermarkets and petrol stations can open, but other “close contact” businesses must shut.

Level 2 – which applies to the rest of the country – is less severe.

People can still go to workplaces and schools, but they must keep their distance and wear a mask if possible. Gatherings of more than 100 are banned.

Aside from the lockdown measures, the outbreak also prompted Ms Ardern to postpone until Monday the dissolution of parliament so that September’s election can take place. In her latest address, she said no decision had been taken on the election 19 September date yet – she would decide in the next 48 hours.

Before August 11, New Zealand was one of only a handful of countries to go so long without recording a locally transmitted case of Covid-19.

All 22 active cases of the virus before August 11 announcement were among returning travelers quarantined in isolation facilities.