The prime minister has confirmed that the New Zealand border will be closed to anyone who is not a New Zealand citizen or permanent resident from 11.59pm tonight.
Children and partners of New Zealand citizens and permanent residents will be allowed to enter.
People from the Pacific will be included in the border closure, as will temporary workers or temporary visa holders such as students.
If travellers are boarding a flight, transiting on their way to New Zealand, or on an aircraft before midnight, they will be able to land and enter the country when they arrive.
"I recognise how extraordinary this is," said Ardern, but the measure had to be taken to protect New Zealand from Covid-19
"In no time in New Zealand's history has a power like this been used."
The ban applies to people not product, and those staffing freight ships and planes are not included.
Those who have travelled here from other countries recently are still required to self-isolate, and those who have been here longer are being encouraged to look at how they can get home.
"I'm not willing to take risks here," Ardern said.
She said the decision was made between 4pm and 5pm this evening. Only Australian Prime Minister Scott Morrison was told of these changes beforehand.
Those who have already boarded or are enroute will be able to enter New Zealand.
"I have simply removed the risk. I'm not willing to tolerate risk at our border, that is where predominantly of our cases are coming from," said Ardern.
She added that while "we will continue to have cases in New Zealand as we continue to test those with symptoms who have come home, we must slow down the transmission in New Zealand".
Earlier, there were travel bans on anyone arriving from China and Iran, and all others - except those from the Pacific - had to self-isolate for 14 days.
Earlier this afternoon, the government announced a ban on indoor gatherings of more than 100 people.
Ardern has already given one update on the government response to the Covid-19 coronavirus today. Speaking in Rotorua, she told New Zealanders they must prepare for the full effects of the Covid-19 coronavirus, but must not panic, especially over rumours and misinformation.
