New Zealand will today unveil a huge economic rescue plan to try to save jobs and businesses, as the Prime Minister warns of drastic action against travellers refusing to self-isolate against coronavirus - and the United Kingdom and New York go into lockdown.

UK Prime Minister Boris Johnson is urging Brits to avoid pubs, clubs and theatres and stop non-essential travel in an effort to reduce the impact of the coronavirus.

The drastic UK action follows New York closing restaurants and bars for dine-in customers - allowing for takeout only - and comes as the New Zealand Government prepares for its biggest day of the crisis so far, with the unveiling on Tuesday of the multi-billion-dollar rescue plan.

Prime Minister Jacinda Ardern has also issued a strong warning to overseas visitors who come to New Zealand and fail to follow self-isolation rules: "Frankly, you're not welcome".

"You should leave before you are deported," Ardern said yesterday, in a warning to those thinking about ignoring the newly imposed arrival restrictions.

In the UK, Boris Johnson said: "Now is the time for everyone to stop non-essential contact and to stop all non-essential travel" as he urged people to work from home and avoid pubs, clubs and theatres.

According to the Scientific Advisory Group for Emergencies (Sage) "it looks as though we are now approaching the fast growth part of the upward curve" in the number of cases, he said.

"Without drastic action cases could double every five or six days," he said.

In New York, New Jersey and Connecticut, gatherings of 50 people or more are banned. As well as the clampdown on bars and restaurants, New York schools are also closed, while court cases have been postponed.

Back in New Zealand, Cabinet has given the green light for the Immigration Minister to be able to deport any foreigners who do not comply with a medical officer's instruction to self-isolate, which will be made a condition of every visitor visa.

As of 1am yesterday, every traveller to arrive in New Zealand from any country, excluding the Pacific Islands, is required to self-isolate for 14 days. Those restrictions will be reviewed in a fortnight but Ardern said they were likely to continue into April.

Ardern also had a message to any symptomatic New Zealanders who fail to voluntarily self-isolate, saying they could be placed in a medical facility with a police officer stationed at the door to prevent them from escaping if needed.

"There will be zero tolerance for those who do not follow the rules of self-isolation," she told post-cab yesterday afternoon.

This comes as New Zealand prepares for, what Ardern described as, "the new normal" when it comes to social distancing.

"If you're close enough for someone to spit on you when you talk, you're too close."

In a bid to further reduce the risk of the disease spreading she said all gatherings of more than 500 people, excluding schools and universities, will be cancelled.

New Zealand's number of confirmed cases remained at eight yesterday although the Ministry of Health said there were two further probable cases.

Just under 3000 people are in self-isolation while a further 7000 have completed self-isolation.

Ardern has warned the economic impact of Covid-19 will be very significant.

She said preliminary advice from the Treasury suggests the economic impact of the virus "could be greater than the impact of the global financial crisis".

The comments come as the Government today prepares to unveil a multi-billion dollar spending package aimed at curtailing the economic damage which is expected to be caused by Covid-19.

Speaking to reporters at post-cab, Ardern's comments focused exclusively on Covid-19 and the Government's response.

She was particularly strong on overseas visitors who may be planning on ignoring New Zealand's self-isolation rules.

The fact that Cabinet had signed off deportation powers reinforces how seriously the Government is about protecting the health of New Zealanders, Ardern said.

"We are a country who takes our roles and responsibility of being hospitable very, very seriously.

"But in return, we ask visitors to reciprocate – we will look after you if you look after us."

Ardern said today's spending package would focus "relentlessly on jobs" and will show the Government is willing to act decisively during these uncertain times.

She has already signalled the cash injection would be the "most significant package" she would announce during her tenure as Prime Minister.

It will contain wage subsidies for Covid-19 hit businesses and employees across the country.

"[It will focus] on supporting businesses to keep people in work as they plan for the future," Ardern said.

It would also focus on ensuring New Zealand's health system has the resources it needs as Covid-19 continues to spread.

"The approach the Government is taking is to ultimately lessen what will be the significant effects [of Covid-19]."

The package comes a day after the Reserve Bank's shock 0.75 per cent cut to the official cash rate (OCR) – a move not seen since the global financial crisis 12 years ago.

The central bank's governor, Adrian Orr, confirmed yesterday the Reserve Bank had modelling to suggest New Zealand was going into a recession.

Earlier in the day, Air New Zealand announced it would cut its international capacity by 85 per cent in the next few months as a result of the impact of coronavirus.

The airline placed itself into a trading halt yesterday to allow it time to more fully assess the operational and financial impacts of global travel restrictions.

And the airline could cut its workforce by nearly a third as it battles to survive plunging demand for air travel.

On current modelling, Air NZ will be looking to reduce staff by up to 30 per cent. That would be 3750 of its 12,500 staff.

Ardern wouldn't comment on Air NZ's financial position or its decision to cut staff to stay viable.

She did say however, there would be "ongoing dialogue".

Meanwhile, Ardern said she had no plans to change the election date of September 19 at this stage.

She also confirmed that she would not attend the Asean trip to Vietnam scheduled for April.

Ministers were giving government departments space to deal with the Covid-19 outbreak, as this was an "all of government" response, Ardern said.

"We are putting huge pressure on our public servants and they are responding magnificently."

She said NZ First MP and Minister Tracey Martin, who had contact with Covid-19 infected Peter Dutton recently, tuned in to Cabinet today via online link.

Ardern said all Ministers should follow the same rules as everyone else.

Martin has not been tested, and Ardern said testing was up to clinicians.