"Their family became concerned about their condition and called Healthline.
"They were advised to seek medical attention and attended Auckland City Hospital emergency department that same day. All were wearing masks on arrival. As a result of the individual's symptoms and travel history they were admitted and tested."
The person had been coughing and had difficulty breathing.
They had tested negative twice but the results of the third test were positive and confirmed at 4.15pm today. It makes New Zealand the 48th country to have a confirmed case of the virus.
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The person is in a stable condition.
"They are in an improving condition in isolation, in a negative pressure room to prevent any spread of the disease," the spokesman said.
They travelled on a New Zealand passport. It's not clear if they went through the E-passport line or saw someone through customs.
Public health officials have begun tracing the person's other close contacts and people who were in same row as the individual or two rows ahead or behind. They will have to self isolate for 14 days.
They were also contacting the flight crew who were in the cabin.
Health Minister David Clark earlier announced a raft of new measures to combat the spread of coronavirus, including new travel restrictions from Iran.
The Ministry of Health says there is a high likelihood of sporadic cases in New Zealand but the chances of a community outbreak remains low, despite the confirmed case.
"The Ministry of Health is confident the public risk from this new infection is being well managed because of the public messaging, awareness of COVID-19 disease and our public health response to managing cases and contacts," the spokesman said.
At a press conference tonight, Director-General of Health Ashley Bloomfield said the person had worn a mask during the flight so it was unlikely they had transmitted it to other passengers.
Anyone who has symptoms and has been in a country with the virus should not turn up at their local GP but phone ahead so the surgery can take the right precautions.
"We have a family that has done exactly the right thing."
The person was put into the isolation room straight away because of the high suspicion they might have the virus, he said. That suspicious was based on symptoms and where they had been.
Earlier in Sydney, Ardern said: "They are obviously being treated with all of the precautions that you would expected and this of course is all subjected to the pandemic plan that we have had in place since early January we have been prepared for this scenarios and we are now in informing all of the protocols we've long had in place for a scenario such as this."
She said the person was a permanent resident in New Zealand and was returning home.
"They had been for a period of travel in Iran but travelled into new Zealand over a period of several days starting their journey on Monday."
She added: "I do want reiterate and acknowledge that it has been through good care and good management that new Zealand has gone through such a long period of time without having a case arrive on our shores throughout that time you would me reiterate that the chances of us receiving a case were extremely likely that is why our pandemic plan has been operationalised for some time and of course now we are utilising that plan in full.
"We were prepared we are prepared this is exactly the scenario we expected would arrive now that it has we are rolling out all of the protocols you would expected."
Auckland Mayor Phil Goff urged New Zealanders to remain calm.
"As the gateway to New Zealand, a case of the coronavirus in Auckland was close to inevitable and health authorities are well prepared to handle it.
"The Ministry of Health advises that the patient is being treated in a negative pressure room to prevent the virus from spreading, and people who were in immediate contact with the patient are also in isolation. The Ministry has said the risk of a community outbreak remains low, and a pandemic plan is in place to manage the situation.
"Health authorities are closely monitoring the situation in line with World Health Organisation guidance and I encourage Aucklanders to remain calm. The Ministry will advise if any public health measures become necessary."
Ireland and the Netherlands have also announced the first cases of the deadly virus in the past 24 hours.
The New Zealand Government will also be bolstering the health presence at international airports and there will be no exemptions for overseas students from China to enter New Zealand, said Clark.
So far, the Government has conducted 130 Coronavirus tests in New Zealand - they have all come back negative.
