New Zealand reported on Thursday its biggest rise in COVID-19 infections in six weeks, with all cases detected in Auckland, raising prospects of a further extension of lockdown restrictions in the country's largest city beyond next week.

Some 1.7 million people in Auckland are under strict stay-home orders until Monday as officials look to stamp out the highly infectious Delta outbreak, the first major spate of community cases in the country since early in the pandemic.

Deputy Prime Minister Grant Robertson said the surge in case numbers in Auckland was not unexpected "but they are rising more quickly," and blamed illegal home gatherings for the spike.

"Now is not the time for complacency," Robertson said during a media conference in Wellington, urging residents in Auckland to strictly follow the level-three rules, under which most people are required to stay at home unless they have urgent reasons to go out.

A total of 71 new local cases were reported in the country, all detected in Auckland, up from 55 a day earlier.

"Today's new case numbers are sobering but not unexpected because of where we are in the outbreak," Director of Public Health Caroline McElnay said.

About 2.49 million New Zealanders have been fully vaccinated, or about 59% of the eligible population, with officials promising to end lockdowns once 90% are vaccinated. Officials are looking to administer a record 100,000 doses in a single day during a mass immunisation drive on Saturday.

Even with the Delta outbreak, New Zealand has recorded only 4,472 confirmed COVID-19 cases and 28 deaths during the pandemic, far lower than many comparable countries.