SYDNEY (REUTERS, BLOOMBERG) - 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.

New Zealand will impose mask rules and limit gatherings from midnight on Sunday (Jan 23) after a cluster of nine cases of the Covid-19 Omicron variant showed community transmission from the North to South islands after a wedding.

A family travelled by plane from the North Island capital of Auckland to a wedding in the South Island attended by 100 people.

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, and a cap of 100 customers indoors in hospitality settings and events such as weddings, or 25 people if venues are not using vaccine passes, Ms Ardern said.

"Our strategy is to slow the spread of Omicron down," she told reporters.

"We know we will see far more cases than we have in the two years of the pandemic to date. But the difference to previous outbreaks is that we are vaccinated and we are even better prepared."

"My wedding will not be going ahead," she told reporters, adding she was sorry for anyone caught up in a similar scenario. She was due to marry long-term partner Clarke Gayford, a television presenter, later this month. The couple have a three-year-old daughter.

Asked by reporters how she felt about her wedding cancellation, Ms 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."

Ms Ardern encouraged more people to get a booster vaccination before Omicron takes hold.

About 56 per cent of those eligible have had the third shot "but we need to get that number higher, quicker", she said.

She outlined three phases in the government response.

Stage one, when there are fewer than 1,000 cases a day, will be a "stamp it out" approach with contact tracing, polymerase chain reaction testing and self isolation.

Stage two will be an adjustment phase when the focus is on those at greater risk of severe illness.

Stage three, when there may be thousands of daily cases, will see changes to contact tracing, shorter isolation periods and application of a test-to-work regime to ensure essential businesses and supply chains can operate.