She made the announcement after nine cases of the Omicron variant were detected in a single-family that flew to Auckland earlier this month.

From Monday, New Zealanders will face strict coronavirus restrictions and the prime minister herself has been forced to delay her wedding to long-time partner Clarke Gayford.

The wedding was expected to take place next weekend at Gisborne on the North Island's eastern coast.

"Such is life," Ms Ardern said.

"I am no different to, dare I say it, thousands of other New Zealanders who have had much more devastating impacts felt by the pandemic, the most gutting of which is the inability to be with a loved one sometimes when they are gravely ill. That will far, far outstrip any sadness I experience."

New Zealand had been among the few remaining countries to have avoided any outbreaks of the Omicron variant, but Ms Ardern acknowledged last week that a new wave was inevitable given the transmissibility of the variant.

The country managed to contain the Delta variant, with an average of about 20 new cases each day but, since it opened its borders in November, it has seen an increase in people arriving into the country and going into mandatory quarantine who are infected with Omicron.

That has put a strain on the quarantine system and prompted the government to limit access for returning citizens while it decides what to do about reopening its borders.

What is the 'red setting'?

The "red setting" of New Zealand's pandemic response includes heightened measures such as mandatory mask-wearing and limits on social gatherings.

Businesses and schools remain open and domestic travel can continue but vaccine passports are required for entry to most non-essential businesses.

Up to 100 vaccinated people can gather under a red alert but the prime minister said her wedding would not proceed in a restricted form.

Ms Ardern stressed that "red is not lockdown", adding: "Our plan for managing Omicron cases in the early stage remains the same as Delta, where we will rapidly test, contact trace and isolate cases and contacts in order to slow the spread."

About 93% of New Zealanders aged 12 and over are fully vaccinated and 52% have had a booster jab.

The country has just begun vaccinating children aged between 5 and 11.