An infectious disease specialist says more people with coronavirus could already be in the community and might be silently transmitting the disease.
Public health professor Michael Baker made his comments after it emerged an Auckland woman with COVID-19 wasn't diagnosed until eight days after she arrived from northern Italy last week. During that time she took a return flight to Palmerston North.
Work is underway to trace passengers who were sitting near her - and anyone else she may have been in close contact with.
Italy is the worst-affected country in Europe, with 107 deaths from the illness reported so far. Many regions remain in lockdown, with schools and universities closed.
The woman arrived before the requirement for travellers from northern Italy to self-isolate was put in place.
Public health professor Michael Baker said similar situations were likely to arise.
"A lot of people have come over to New Zealand from overseas obviously in the last six weeks or so and some have come from places where there is active transmission," he said.
"Many will self-isolate as a form of quarantine, which will greatly reduce the risk. But there will be people who have got through the system that we've identified as being at moderate risk of being affected.
"We could already have community transmission happening in New Zealand. The term used quite a bit is silent transmission and that is because this is a mild illness for most people, maybe up to 80 percent and so they can have what looks like a common cold and can infect other people.
"The transmission doesn't become obvious until someone is infected and gets quiet ill and presents to the health system, sometimes with pneumonia and sometimes admitted to hospital. So that's also transmission and that's what's happening already in quiet a few overseas countries... That's the thing we have to be on the look-out for in New Zealand now."
It should be known on Thursday whether the infected woman's partner also has the coronavirus, which would bring the number of confirmed cases in New Zealand to four.
The Auckland family - including two pupils at Westlake high schools - are in isolation at home, with the man showing signs of illness.
The family has been the target of abuse on social media and authorities have called for calm and restraint.
Baker said every day allowed the country to ramp up its preparation for what would probably be a very widespread pandemic situation in New Zealand.
He said New Zealanders must exercise caution and take responsibility, and avoid feckless behaviours like soldiering on through illness to go to work.
"If you have symptoms, don't work, don't go to school, don't go to social gatherings and infect people... This is a potentially fatal condition that you are transmitting, so people really need to take responsibility for this.
"Some point over the next few weeks and months we will have more and more cases in New Zealand and so I think we just need to develop a new habit and culture around how we manage illnesses."
Meanwhile, half a dozen people are due to be released on Thursday after being quarantined for two weeks on a Defence Force base.
They were passengers on the Diamond Princess cruise ship where more than 700 people were infected with coronavirus in Yokohama, Japan.
Eight health officials who visited the ship were also infected.
The six New Zealanders were sent to the Whangaparaoa military base in northern Auckland to check they were clear of the disease.
Officials say all six were in good spirits before their final health checks this morning.
An earlier intake of 157 people evacuated from China were released on February 19.