Prime Minister Jacinda Ardern and Finance Minister Grant Robertson head into a Monday press conference to announce New Zealand would shutdown.

Prime Minister Jacinda Ardern says families who live apart can consider themselves part of a tightly-held "bubble" that should isolate from others for the next month.

Ardern sought to further explain how the self-isolation of New Zealand's whole population will work during a press conference on Tuesday.

"Whatever your bubble is for the month, this is the bubble you must maintain. That has to be it, a small group of individuals," she said.

"As soon as you start opening up contact with different individuals, if that person did the same with their different individuals, that's where the risk grows. So I'm asking people to apply common sense."

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New Zealand is preparing for a shutdown for four weeks, a severe measure hoped to break transmission of the virus, due to begin after a deadline set at 11.59pm on Wednesday night.

"The simplest thing New Zealanders can do to stop the spread of this virus is to stay at home. That is how we will save lives.

"Isolation and physical distancing from other people is key to our response at this level."

Ardern said she wanted to assure New Zealanders the Government was protecting their public health with urgency.

There were still some logistical issues to be ironed out but the Government was take a common sense approach.

The Government would make sure people could get home if they were at a ferry terminal or needed a domestic flight to get home, she said.

"We will work calmly and sensibly though the issues as we go. But I simply wasn't willing to wait until we could answer every single question on some of these logistical challenges before we moved to level four.

"Every day we waited would have been more time that Covid-19 could spread in our community. We will work thought this as we transition," she said.

KEVIN STENT Dr Ashley Bloomfield, Director-General of Health, left, on Tuesday confirmed New Zealand had 155 cases of Covid-19.

People needing to travel on domestic flights, trains and Cook Strait ferries to get home will be able to continue using the passenger services until midnight on Friday, Transport Minister Phil Twyford confirmed on Tuesday evening.

Domestic passenger services, particularly ferries, have been inundated with people trying to get home before the original cut-off date of midnight Wednesday.

"The Government recognises New Zealanders are trying to do the right thing by returning home ahead of the country moving to level 4," he said.

Non-essential workers and international tourists can keep using domestic passenger services until midnight Friday. After then, services will only be provided to essential services and workers, including freight services.

International travellers will only be allowed to use domestic passenger services to reach their final destination to comply with lock-down requirements.

People have been ordered to stay at home and self-isolate from all others. Schools and non-essential businesses have begun to close. Travel around the country would be "severely limited" for at least four weeks.

On Tuesday, New Zealand had a total of 155 cases of Covid-19, a jump from 102 cases the day prior.

Ardern said the social isolation "bubble" people were expected to maintain could apply for people in relationships but who lived in separate homes, or shared custody of children.

Such a "bubble", that would be larger than a single household, should be tightly held.

"Apply common sense, keep your distance from everyone outside of your bubble," she said.

The advice echoed that of the secretary of education Iona Holsted who said essential workers, 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 schools were shut.

This person would become part of their tight circle for self-isolation, Holsted said.

Ardern on Tuesday also said travellers who are stuck in New Zealand due to travel routes drying up will have their visas extended.

There would be a further update on the essential services that would remain open, tomorrow, and this may change as the month-long lockdown progresses.

"The more businesses that stay open, the more chances the virus has to spread," Ardern said on Tuesday.

Ardern said New Zealand could show its gratitude to essential workers, who continued to go to work as the virus spread, by staying home.

"Every interaction we have with someone else increases the risk of the spread of the virus."

Finance Minister Grant Robertson, also at the Tuesday press conference, said the major retail banks had agreed to a six-month mortgage holiday for people who had their income affected due to the economic affects of Covid-19.

The exact criteria was yet to be finalised, but it will cover both principal and interest payments. Robertson said he urged people not to "bombard" banks with queries about having a mortgage holiday.

Businesses will also be covered by a $6.25 billion finance guarantee scheme, being organised by both government and the banks, that is hoped to save jobs and protect businesses.

"We do believe it will support a number of mid-range business to allow them to continue," Robertson said.