New Zealand Prime Minister Jacinda Ardern was Sunday forced to call off her own wedding as she tightened Covid-19 restrictions in the face of an outbreak of the Omicron variant.

"My wedding will not be going ahead," she confirmed after detailing new restrictions; including a limit of 100 fully vaccinated people at events.

"I just joined many other New Zealanders who have had an experience like that as a result of the pandemic and to anyone who is caught up in that scenario I am so sorry."

The emergence of nine cases of Omicron in a family who travelled between cities to attend a wedding, and the infection of a flight attendant on one aircraft they flew on, forced New Zealand to impose its "red setting" restrictions from midnight Sunday.

Omicron is far more transmissible than the earlier Delta variant but is less likely to make people seriously ill.

In addition to limiting crowd numbers, face coverings are now mandatory on public transport and in shops.

Ardern and long-time partner Clarke Gayford have never announced their wedding date, but it was believed to be scheduled for some time in the next few weeks.

The new restrictions are set to remain in place until at least the end of next month.

"Such is life," Ardern said when asked how she felt about setting regulations which ended her planned nuptials.

"I'm no different to 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 outstrip any sadness I experience."

New Zealand has detected 15,104 Covid-19 cases and recorded 52 deaths since the pandemic began.

Tough border restrictions and snap lockdowns have been in place for much of the last two years.