Prime Minister Jacinda Ardern on Monday extended a lockdown in New Zealand's largest city, saying more time was needed to ensure a stubborn cluster of coronavirus infections was under control.

Ardern said stay-at-home orders would remain in place across Auckland until at least late Sunday, instead of restrictions easing late Wednesday as originally planned. "These extra four days are believed necessary to allow us to move down a level in Auckland, and stay down," she said. "We want both confidence and certainty for everyone."

Auckland went into lockdown on August 12, a day after the virus re-emerged in the city, ending New Zealand's record of 102 days without community transmission. The cluster has since grown from four to 101 and its source remains unknown, despite extensive genome testing and contact tracing.

Ardern said more cases were likely and health authorities needed to confirm the cluster was contained before changing the lockdown settings. She said even when the Auckland restrictions are eased, the entire country could remain at Level Two, the second-lowest rating on New Zealand's four-tier virus alert system.

It means social distancing and limits on mass gatherings until at least Sunday, September 6. Ardern also said masks would be compulsory on public transport and flights. New Zealand suppressed the virus with a strict seven-week lockdown earlier this year and its re-emergence has forced the country's general election to be pushed back to October 17 from September 19.