New Zealand has 40 new cases of coronavirus, more community transmission of the virus, and three new probable cases.

Probable cases are now being included in the total count, meaning New Zealand now has 155 cases of Covid-19, including 142 cases that have tested positive.

Four of the cases are now considered to be the result of community transmission, three in Auckland and one in Wairarapa.

The number of cases has been expected to climb, as Kiwis flock home from virus-stricken countries and community transmission is rooted out.

ROSS GIBLIN/STUFF Travellers waiting to get on the InterIslander ferry before non-essential travel is banned on Wednesday.

READ MORE:

​* 36 new coronavirus cases

* Coronavirus: Full coverage

* Close schools now, says Teaching Council

* Coronavirus police taskforce

* Work from home starts now

Director-General of Health Dr Ashley Bloomfield said, at a press conference on Tuesday, that 12 people had recovered from coronavirus, and six people were in a stable condition in hospital - meaning they have not needed intensive care beds.

Four cases were being treated as community transmission. Bloomfield said overseas travel continued to be the main source of new cases of coronavirus.

More than 900 tests for the virus were run on Monday. A total of 8300 tests had now been conducted.

He said none of the confirmed cases were healthcare workers, as far as he was aware.

He said the probable cases would now be included in the total coronavirus count. In these cases, a clinician would have diagnosed a person as being probable despite a failed test due to having an exposure history and symptoms.

Prime Minister Jacinda Ardern on Monday announced New Zealand would go into lock down for four weeks, the most severe measure, level 4, of a Covid-19 threat alert system, from the deadline of 11.59pm on Wednesday night.

People were being ordered to stay at home. Schools and non-essential businesses would close. Travel around the country would be "severely limited".

During the Tuesday press conference, director-general was joined by a raft of senior public servants tasked with arranging the shutdown.

Secretary of education Iona Holsted said essential workers who were parents, such as doctors and nurses, could arrange for another person - a "trusted buddy" who was not elderly or vulnerable - to help care for their children while they were not at school.

"Some people will need help," Holsted said. It was crucial this "buddy" would then only have contact with their household, meaning both households become, in affect, one self-isolated group.

Consistency was key to ensure people maintained a tight circle of people who cared for a child, and reduce possible transmission, she said.

SIMON O'CONNOR/STUFF City Care pharmacy in New Plymouth had a limit of two customers in the store at a time, as New Zealand practises social distancing to avoid transmission of coronavirus.

The ministry had sourced some devices, such as laptops and tablets, so that children without these could still learn at home.

But delivering the devices to the childrens' homes was a logistical challenge, and in some cases there would be no internet connection.

The ministry was working with internet providers, television and radio, to determine how they could "beam" into the homes of children.

"Many, many children in this country suffer from poor learning outcomes because of where they live ... this actually might be an opportunity to give them more resources," she said.

KEVIN STENT/STUFF Members of the all of Government COVID-19 national response team, from left, Dr Peter Crabtree, government controller John Ombler, Director General of health Dr Ashley Bloomfield, director of Civil Defence Sarah Stuart-Black, and police Commissioner Mike Bush.

Ministry of Business, Innovation, and Employment Paul Stocks said they were working through which businesses fitted within the essential service category, and a phone line would open up at 5pm Tuesday for businesses to call and check.

Despite the Warehouse announcing it would remain open, the government has not decided it would be classified as an essential service and therefore be permitted to open.

A similar decision was yet to be made about liquor stores.

"If you are unsure about whether you are an essential service, you are probably not," he said.

Controller of the all-of-government response, John Ombler, reiterated that Kiwis must stay home from Wednesday night.

The spread of coronavirus will determine how long New Zealand will be locked down for.

On Monday, 36 new cases were confirmed, bringing the country's total to 102, and two cases were being treated as community transmission.

Community transmission means the person had contracted the illness from someone in the community that cannot be linked to overseas travel.

Most cases have been linked to overseas travel. Cases now include a child and two teenagers, and have been found in almost all parts of the country.