New Zealand Prime Minister Jacinda Ardern has cancelled her wedding as the nation imposes new restrictions to slow the community spread of the Covid-19 Omicron variant, she told reporters on Sunday.

New Zealand will impose mask rules and limit gathering from midnight on Sunday after a cluster of nine Covid-19 Omicron cases showed community spread from the North to South islands after a wedding.

A family returned to Nelson in the South Island by plane after attending a wedding and other events in Auckland in the North Island. The family and a flight attendant tested positive.

New Zealand will move to a red setting under its Covid-19 protection framework, with more mask-wearing. Indoor hospitality settings such as bars and restaurants and events like weddings will be capped at 100 people.

The limit is lowered to 25 people if venues are not using vaccine passes, Ardern said.

Read more: New Zealand PM Jacinda Ardern plans summer wedding

"My wedding will not be going ahead," she told reporters, adding she was sorry for anyone caught up in a similar scenario. Ardern had not disclosed her wedding date, but it was rumoured to be imminent.

Asked by reporters how she felt about the cancellation of her wedding to longtime partner and fishing-show host Clarke Gayford, Ardern replied: "Such is life."

She added, "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's borders have been shut to foreigners since March 2020.

The government pushed back plans for a phased reopening from mid-January to the end of February out of concern about a potential Omicron outbreak as in neighbouring Australia.

People able to travel to New Zealand under narrow exceptions must apply to stay at state-managed quarantine facilities. The government last week stopped issuing any new slots amid a surge in the number of people arriving with Omicron.

About 94 per cent of New Zealand's population over the age of 12 is fully vaccinated and about 56% of those eligible have had booster shots.