And Jacinda Ardern is also making clear that if we thought things would return to normal soon, think again: the whole world's borders could stay shut for a lot longer.

Four weeks in lockdown will be hard enough but prepare for longer and don't plan any overseas travel for up to a year and a half - that's the advice of scientists advising the Prime Minister.

The scientist advising the Prime Minister is tonight warning we should prepare to stay in lockdown for longer than four weeks.

When Newshub asked if there are contingency plans if the country has to go in lockdown beyond four weeks, the Prime Minister said, "many of the plans we have in place have the ability to continue on," she said on Friday.

The four-week nationwide lockdown was effectively the Prime Minister telling the country, 'you're grounded', and if Kiwis misbehave, the Government will be forced to ground us even longer.

"The best chance we have of limiting the time that we're in this lockdown is if we obey the rules while we're in it," she said.

Professor Shaun Hendy, one of the scientists providing modelling to the Government about the coronavirus, is going harder - he says prepare to lockdown for longer - and perhaps not just this once.

"Think about it being four weeks but prepare for six weeks," he told Newshub. "I think it's actually reasonably likely that there might have to be more lockdowns but hopefully regional lockdowns."

That could mean future lockdowns if New Zealand sees cases flare-up again or an extension of the current lockdown if we don't see the case numbers turn around

Director-General of Health Dr Ashley Bloomfield says it will take "between a week and 10 days" before we start to see the numbers turn around, confirming on Friday that New Zealand has 85 new cases of the virus.

Prof Hendy said by the time New Zealand goes into week three of the lockdown, which he's been told is going to be the toughest week psychologically for people, we should have an idea of how long the lockdown will last.

All travel overseas for Kiwis is going to be restricted for quite some time.

It was just eight days ago the Prime Minister took the sudden move to slam the border shut, and opening them back up would take far longer.

"It's going to be border lockdowns until a vaccine comes along," says Prof Hendy.

Newshub put that to the Prime Minister she said: "the border right now does present the greatest risk for us managing COVID-19, both now and on an ongoing basis".

Ardern says she is "very determined" once the Government gets the virus under control to "keep it under control".

Work on a vaccine is underway but could take 12 to 18 months, and Hendy is warning Kiwis not to plan overseas trips for the next year to 18 months.

And don't plan on welcoming people home from overseas either.

"The border will be an ongoing risk that we will need to manage, in fact, that the whole world will need to manage," Ardern said.

If New Zealand stamps the virus out, there's no way we want it back, so long-term border restrictions could be a reality.

And even if the lockdown lifts in four weeks, it could just be a shift from alert level four to three - which is still restrictive, with businesses closed and Kiwis working from home.