A third case of coronavirus has been confirmed in New Zealand.

Health minister Dr David Clark told the AM Show this morning he had received a report of a third positive test of Covid-19.

He did not know where the person was in New Zealand but said their case was not related to the second case of Covid-19 in New Zealand, which was confirmed yesterday. Further information would be issued at a media update this afternoon.

The case comes just one day after a woman in her 30s was confirmed to have the virus after arriving back in New Zealand on February 26 from northern Italy, where there is an outbreak.

That woman then took two domestic flights between Palmerston North and Auckland on March 2.

She also visited two medical centres before being diagnosed. Health authorities are contact tracing for the woman, who is in self-isolation.

Her partner is also in self-isolation and is displaying symptoms - his test for the virus is expected back later today. He is not the third confirmed case, Health Minister Dr David Clark told the AM Show.

Her two children - who attend Westlake Boys' and Westlake Girls' High School - are also in self-isolation but are not displaying any symptoms.

The first case was confirmed on Friday February 28 after a person in their 60s returned to New Zealand from Iran via Bali. They felt unwell and wore a mask on their flight before heading to Auckland City Hospital where they are in isolation.

Bloomfield told Newstalk ZB's Mike Hosking this morning that person is stable and continuing to improve, but being sent home from hospital is "not on the horizon".

He said children from Westlake Boys' and Westlake Girls' High Schools were not at risk from the virus.

"The lowest attack rate seems to be in people under 18 and furthermore, kids get a mild illness if they get it at all."

Neither is displaying symptoms of the virus and it is rare for asymptomatic people to spread the illness, according to the World Health Organisation. The virus is normally transmitted by people coughing or sneezing.

In the past day the WHO's Director-General Dr Tedros Adhanom Ghebreyesus has said the global mortality rate was 3.4 per cent - higher than the 2 per cent that has been widely cited.

However, Bloomfield said he was not too concerned about that number as it was a "crude rate" based on the number of deaths divided by the number of reported cases. There were likely to be many additional unreported cases.

Speaking to ZB's Kate Hawkesby this morning, Otago University Professor of Public Health Michael Baker said people should be ready for a community outbreak - and should start staying home if they have respiratory illness.

Baker has previously expressed concerns at New Zealand's lack of preparedness for a pandemic, but he said the current approach to the virus was "very reasonable".

That meant banning arrivals from certain places and requiring self-quarantine from others.

But he said people should be ready for the disease to move to a community outbreak phase - and at that point the response should be very different.

"We have to now start planning for the idea that there will be at a certain point a move into the management phase when containment is no longer possible and you have community transmission."

While he said it was possible nobody in New Zealand had yet been infected, people with respiratory illness should be staying home from work and school and not socialising.

Data out of China showed that there were three key symptoms of Covid-19 - fever, cough and fatigue (feeling unwell and weak).

"We have to get in the habit now, if you develop those symptoms, regardless of if you've been overseas or in New Zealand - just stay at home."

Baker said people's panic buying was understandable, given the scenes of chaos overseas.

"The message now is prepare, don't panic. That's easy to say, everyone has varying levels of anxiety. But the thing that will get us through this is ... just good preparation, listening to instructions and information coming out of the Ministry of Health - they are really doing an extremely good job on this."