Key points:

- Four-stage alert system for COVID-19 announced
- New Zealand moved up to COVID-19 Alert Level 2 – Reduce Contact
- New Zealanders over 70 and those with certain medical conditions told to stay at home as much as they can to reduce risk of contact with the virus
- Workplaces to implement plans to reduce person-to-person contact, including work from home where possible
- Limit all non-essential domestic travel

Prime Minister Jacinda Ardern has introduced an alert system as the country fights Covid-19.

She has addressed the nation about the impact of the coronavirus on New Zealand and what will happen next.

The goal was to slow down a "tidal wave" of Covid-19 and break it into smaller waves, which would reduce the impact on health and the economy.

"It does mean we have to be ready to step our action if we need to."

She said New Zealand had warning systems, and an alert system is now in place for Covid-19 that can apply to the whole country, or to certain places or towns.

Ardern said at all levels of the alert system, essential services would still run.

She said there four alert levels to the system.

One was where Covid-19 is here but contained.

Two was where Covid-19 is contained but the risks are growing as more cases arise, and unnecessary travel should be contained and more travel restrictions are put in place.

Three and four are where community transmission is more widespread, and contact with people is more restricted with communities in lockdown.

Alert level two is where New Zealand currently is, she said.

The risk of transmission was growing, she said.

People over 70 or with compromised immunity should stay at home as much as possible, Ardern said.

Friends and neighbours need to support people in this group, she said, by dropping off supplied where possible.

Workers also needed to start working differently.

She said people would not always be able to work from home, though workers in essential services would still need to work on site.

Physical distancing should be used, and people should limit their travel around the country, she said.

"Every unnecessary movement gives Covid-19 a chance to spread."

At alert level two, Ardern said schools would be closed if a case affects a school.

"We are constantly monitoring these settings."

She said it was a time where people needed information, and people should look at Covid19.govt.nz to see official information, and be wary of misinformation.

"Please do stay tuned, and we will share daily updates."

"I ask what New Zealand does what we do so well. We are a country that is creative and community-minded.

"We know how to rally. We know how to look after one another.

"Be strong, be kind, and unite against Covid-19."

Closing the borders seemed unimaginable a month ago, but now seemed to be an obvious step to take to combat the outbreak, she said.

The vast majority of people with Covid-19 will only have mild to moderate symptoms, but others will need care.

Ardern said she was speaking to all New Zealanders to give them as much clarity and certainty as the world rapidly changes.

This is the first time Ardern has made an official address to the nation.

It will be followed by a press conference from the Beehive with Ardern and Director General of Health Ashley Bloomfield.

New Zealand now has 53 confirmed cases of Covid-19, including two cases which have no link to overseas travel.

"At this point, we cannot rule out a risk of community transmission in these [two] cases," said Director-General of Health Ashley Bloomfield told a press conference on Saturday morning.

"We always knew that cases not linked to travel could happen and we are prepared for that," he said.

A further 14 positive tests were confirmed in the last 24 hours, the biggest jump in cases in a single day so far.

The two cases of Covid-19 that may have come from community transmission are in Auckland and the Wairarapa.

While the borders are closed to non-citizens and non-residents and social gatherings have been curtailed, Ardern has not forced a lockdown on any community.

She has previous said that a lockdown would only occur to combat widespread community transmission.

The two cases were part of 14 new cases in the past 24 hours, bringing the total number of confirmed cases in New Zealand to 53. Three of those cases were in hospitals and in a stable condition in Queenstown, Nelson and the North Shore.

Bloomfield said that half of the 56 New Zealanders on the cruise ship Ruby Princess - where four people had tested positive for Covid-19 - had returned to New Zealand, and all of the New Zealanders were being contacted.

But they would not be tested unless they showed symptoms.

Another cruise ship, the Celebrity Solstice, had an Auckland man in his 60s on board who had tested positive for Covid-19.

He joined the ship in Auckland on 10 March and departed the cruise in Dunedin after visiting Tauranga (March 11), Picton (March 13), and Akaroa (March 14.

The ship arrived in Sydney this morning and any New Zealanders on board returning to New Zealand will be regarded as close contacts, put in self-isolation and monitored daily.

Ardern's address today comes as the global pandemic continues to march on, with London and the United Kingdom going into lockdown and workers in major states and cities in the United States told to stay at home.

As the number of confirmed cases rises in New Zealand, libraries, art galleries, museums and university lecture theatres are all temporarily closing their doors.

New Zealand closed its borders to non-residents and non-citizens on Friday, and other countries have gone into lockdown with citizens told to return home. Italy, with 60 million citizens, has recorded 3405 deaths, exceeding the 3248 in China, a country with a population 20 times larger.

It took three months to reach 100,000 coronavirus cases worldwide. The second 100,000 took only 12 days.

Even more major sporting events have been cancelled as health officials tell New Zealanders to practice "physical distancing" whenever they can.

But Ardern has reiterated her call that people should not panic and says Kiwis should "trust doctors".

The Government yesterday announced details of a $900 million loan to embattled Air New Zealand.

And the New Zealand Superfund revealed it has lost almost $9 billion since the start of the year and supermarkets place temporary limits on some items in store amid "unprecedented demand".

Leaders in both Auckland and Wellington have moved to temporarily close public places to prevent the virus' spread.

Auckland's art galleries, pools, recreational centres and libraries would all be closed for at least two weeks in response to Covid-19, Mayor Phil Goff confirmed yesterday morning.

This was, as he said, a "necessary step".

He said the moves would not result in any job losses but staff at the public facilities could be redirected into other roles related to the Covid-19 response.

In Wellington, the Te Papa's chairwoman Dame Fran Wilde announced the museum would close for at least two weeks, due to Covid-19.

It was a similar message from Auckland University Vice-Chancellor Dawn Freshwater, who confirmed the university would suspend teaching across its campuses next week.

The university's intention is to be in full digital teaching and learning mode from the week of March 30 onwards.

British Prime Minister Boris Johnson announced the shutdown of London and the UK today.

All pubs, bars, shops, restaurants, gyms and leisure centres across the UK will shut their doors tonight as the Prime Minister unveiled a package of measures to protect workers and the economy.

Johnson urged people not to go out tonight as he introduced the stringent measures, which he said were necessary to "turn the tide" on the epidemic.

The UK Government has also announced "an unprecedented" plan to pay 80 per cent of wages in order to protect workers. The scheme will cover the salary of those on up to £2500 ($NZ5100) a month, just above the median income.

As the number of new UK infections today leaped by 714 up to 3983, Johnson said that 75 per cent of the population should stay indoors.