International visitors to New Zealand must self-isolate for 14 days upon arrival to help slow the spread of the new coronavirus, Prime Minister Jacinda Ardern said.

Ms Ardern said the "unprecedented and far-reaching" measure would come into effect from midnight on Sunday and affect all inbound travelers, except those from Pacific island nations.

She said remote New Zealand had been relatively unscathed by coronavirus so far, with six confirmed cases and no fatalities, but the evidence showed the numbers would inevitably rise in the face of a global pandemic.

"But the scale of how many cases we get and how fast we get them is something we should do as much as we can to slow," she told reporters. 

"We must go hard and we must go early."

Ms Ardern said the self-quarantine order would be reviewed after 16 days.

She said New Zealand had also told cruise ships not to visit the country until at least 30 June, while New Zealanders were being advised to avoid all non-essential travel overseas.

"There's no doubt this will have a significant impact on New Zealand's economy but our priority is to ensure we reduce the impacts of COVID-19 as much as we can," she said.

New Zealand has already cancelled several major events, including a memorial planned for tomorrow to mark the first anniversary of the Christchurch mosque shootings in which 51 people died.

"This is a pragmatic decision. We're very saddened to cancel, but in remembering such a terrible tragedy, we shouldn't create the risk of further harm being done," Ms Ardern said.

"The advice we received for this event is that based on people travelling from different parts of the country and from overseas, if there was a case, it could be difficult to trace those who had come into contact with that person, so we are taking a cautious approach.

"March 15 now becomes an opportunity for every New Zealander to reflect in their own way on the events of a year ago."

Ms Ardern said more general restrictions on public gatherings would be imposed but details had not yet been finalised.

Pacific island nations have not yet recorded any confirmed cases of COVID-19, with the exception of French Polynesia, where one person has tested positive. 

Air New Zealand said it was reviewing the impact of the measures announced "and will adjust its capacity accordingly".

The flag carrier has already scrapped its earnings guidance amid plummeting demand.