Prime Minister Jacinda Ardern has warned New Zealanders should get used to border restrictions in New Zealand and overseas, saying they're likely to be in place "for some time".

She said border restrictions overseas would likely persist until a vaccine for the novel coronavirus, believed to be one year to eighteen months away at the earliest - some vaccines take a decade to develop.

"We will be having to manage covid-19 for months, until of course there is a vaccine and that will be many months," she said.

Ardern told RNZ: "I'm anticipating border restrictions for some time."

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KATHRYN GEORGE/STUFF The coronavirus will affect our borders for a long time, the prime minister says.

New Zealand's border is currently closed to all but New Zealand citizens or permanent residents, all of whom must go into self-isolation or quarantine when they return.

This will not be welcome news to tourism operators, who will be faced with months, and possibly years of disruption.

Ardern said ministers were looking at ways to revive the tourism sector as things slowly return to normal.

"We have to think what the future looks like for sectors that have relied on people moving around the globe," she said.

"I am thinking and ministers are thinking about our tourism industry in the long term."

Ardern also braced New Zealanders that the return to normalcy might take some time, signalling the likelihood that even after the four weeks of lockdown only parts of the country would be moved down the alert system.

She said that her "goal" was to "be in a position at least in some parts of the country to move to a lower level of our alert system".

This would mean some or most of the country remaining in lockdown for longer than four weeks.

New Zealand's alert system allows the Government to impose different levels of alert on different parts of the country, depending on the severity of the threat.

Moving parts of the country down to a level three alert, for example, would relax restrictions although most non-essential businesses would still be asked to stay shut, educational facilities and public venues would be closed, and domestic travel restricted.

KEVIN STENT/STUFF Prime Minister Jacinda Ardern warned New Zealanders that border restrictions would be in place for some time.

Other parts of the world are also grappling with the long-term effects of Covid-19, the UK's deputy chief medical officer warned Britons overnight that it would take six months to know whether measures taken to quash the virus's spread had been effective, although this did not mean a lockdown would necessarily last that long.

Economists are busy forecasting the impact of these emergency measures on the economy. ANZ last week forecast GDP to fall 5 to 6 per cent over 2020, with most of the pain yet to come.

The bank put together a gloomy forecast, estimating the likely hit to GDP should the level 4 lockdown last for a full 12 week quarter. This would mean a 30 per cent contraction in GPD in the second quarter of this year.

Director General of Health Ashley Bloomfield also warned the number of Covid-19 cases would likely rise.

"The infections that are diagnosed today are people that were infected, five, seven, or ten days ago, so that's why we will expect the numbers to continue increasing for a while yet," he said.