The third confirmed case of coronavirus in New Zealand appears to be a fourth case, but officials won't say it.

On Thursday afternoon, Ministry of Health director-general of health Dr Ashley Bloomfield explained the third confirmed case was a New Zealand resident who had contracted Covid-19 after family members returned home to Auckland from a trip to Iran.

The man in his 40s was not connected to either the first or second confirmed cases, suggesting he likely contracted the virus from a family member, in what was the first transmission within New Zealand.

But Minister for Health David Clark would not confirm another member of the Auckland man's family had the coronavirus. It was initially said his family members were not being tested, but Clark later said the person who had come back from Iran had been offered and is taking the test.

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"My understanding is that person is asymptomatic and is being treated in self-isolation as if they are a case to ensure there is no risk of further spread," Clark said following Bloomfield's briefing.

The testing would be a "belt and braces approach," he said.

"The best evidence we have at this stage is a large study done in China that shows people who are asymptomatic are not able to spread the virus."

ALI SHIRBAND/MIZAN NEWS AGENCY VIA AP A paramedic tests blood at a hospital in Tehran, Iran. Family of the man diagnosed in New Zealand had returned from Iran recently.

People should be unsurprised by the first person-to-person transmission because evidence was emerging from around the world that the most common cases of transmissions was within families, he said.

On Wednesday night, health authorities notified the man and his family of the result and started the process of contact tracing. The Ministry had learned he had coronavirus at 6pm.

He was in self-isolation at home with appropriate clinical support from public health and district health board staff, Bloomfield said.

Self-isolation at home was an appropriate response for people with mild to moderate symptoms and is recognised by the World Health Organisation, he said.

The man didn't require hospital level care and neither did anyone else in the home. Three other members of the man's family had previously been unwell, none seriously.

They have now fully recovered and are with him in self-isolation. This is a precautionary measure. No one else in the family is displaying symptoms presently, Bloomfield said.

Public health officials have begun tracing the other close and casual contacts of the three family members who had been unwell to ensure appropriate protection measures were in place.

That included the flight on which two family members recently arrived in New Zealand from Iran. Those flight details were: Qatar QR0920 Doha to Auckland, departed 22 February, arrived Auckland 23 February.

ROSS GIBLIN/STUFF Ministry of Health's Director-General of Health Dr Ashley Bloomfield gives an update on the third case of coronavirus in NZ.

In addition to proactive tracing of close contacts on this flight by public health staff, contact would be made with everyone on the flight to provide relevant information.

Bloomfield reminded people that close contacts were people who were in the same row of seats, the two rows in front and the two rows behind.

Healthline knows the seat numbers and will be able to advise anyone on the flight, whether they were considered a close or casual contact.

Anyone who was on the flight and was concerned or would like information was asked to contact Healthline on 0800 358 5453. Interpreters were available.

MEDICAL CENTRE

Contact has been made with the medical centre where the man sought advice. One staff member who treated the man was being treated as a close contact and would be in self isolation for 14 days.

Public health officials were tracing patients who may have had contact with the man, who was symptomatic when he visited the centre.

RICKY WILSON/STUFF Jacinda Ardern talks to the media after NZ's third coronavirus case was confirmed.

SCHOOLS

Two schools have also been notified about the third positive test as there was a family member at each of those schools; Auckland Grammar and Ormiston Junior College.

The students who attend these schools were now at home in isolation. They had no symptoms and there was no risk to students or staff in either of the schools, Bloomfield said.

The two students have not been symptomatic while they have been at school, and were not considered infectious now. The Ministries of Education and Health would work together to provide the schools with appropriate advice and support.

Bloomfield concluded by saying: "I want to repeat what I said yesterday: these are Kiwi families affected by a virus that is part of a worldwide outbreak. What they need is support and understanding. Our task is to ensure they have all the support and healthcare they need."

CHRIS MCKEEN/ STUFF Prime Minister Jacinda Ardern igives an update on the coronavirus situation in New Zealand.

ARDERN: HYGIENE FIRST

At an earlier press conference on Thursday, Prime Minister Jacinda Ardern reminded people about the importance of good hygiene - such as hand washing, and coughing and sneezing into your elbow.

"The same things we advocate every flu season are as important now as before," Ardern said. "The most important thing people can do, is if they are sick stay home."

Ardern reminded people that "basic public health is what's going to get New Zealand through".

A Ministry of Health spokesman confirmed that a positive test had came through on Wednesday night. On Thursday, he told Stuff​ there was already a lot of work underway around the case and processes were moving.

The latest positive Covid-19 result was not related to the partner of the Auckland woman who tested positive on Tuesday.

Starting from Thursday, the ministry will give a daily update on testing each afternoon. This is where it will confirm any new cases from now on, the spokesman said.

Ardern explained there was a process to adhere to, saying the most important thing for us to do is first contact the person and then contact anyone they have been in contact with, and then release this to the public.

"I have every faith in our system. Yes this is a global issue we will have cases and will have to continue to manage those but we are able to do it better than most. "

ROSS GIBLIN/STUFF Minister of Health David Clark.

FIRST TWO CASES

The woman in her 30s - the country's second confirmed case - was in self-isolation with her partner and two high-school aged children who were not symptomatic.

The country's first case of the Covid-19 virus - a woman in her 60s - was confirmed by the Government last week. That person was a citizen of New Zealand who had been to Iran and travelled back to New Zealand via Bali last week.

She is in a stable condition, recovering in isolation at Auckland City Hospital.

WHAT YOU NEED TO KNOW

Public health officials are contacting a number of people who may have come into close contact with the infected patients while travelling on flights.

This includes a flight the first infected woman was on that originated in Tehran and came via Bali.

Any traveller who was on the final leg of the flight, Emirates EK450 arriving Auckland on Wednesday February 26, and was concerned should contact the Healthline number 0800 358 5453.

The second woman who is infected flew on an Air New Zealand flight from Singapore to Auckland on February 25.

The people who were in the same row as the woman on flight NZ0283, as well as the two rows ahead and two rows behind her, would be contacted by public health staff.

Further to this, the woman also took two domestic flights. She travelled to Palmerston North on flight NZ5013 on March 2 and returned to Auckland on flight NZ8114 that same day.

Close contact tracing was also taking place for the people on this flight, and also for two medical centres the woman sought treatment at.

People showing symptoms who have recently been to mainland China, Iran or were on any of the flights - related to the first two confirmed cases - or have been in close contact with someone confirmed to have Covid-19 should phone Healthline on 0800 358 5453.

RNZ Director-General of Health Ashley Bloomfield says he got a report of a third positive test on Wednesday evening.

GLOBAL SITUATION

As of Thursday, there were more than 93,000 confirmed cases globally, and 3198 deaths. Almost 90 per cent of the cases have been in China, mostly from one province.

Outside China, a total of 12,668 cases of Covid-19 have been reported to WHO from 76 countries, with 214 deaths.