Prime Minister Jacinda Ardern has a simple message for Kiwis - "Act like you have Covid-19" and stay at home.

"Breaking the rules could kill someone close to you."

It could also see the lockdown period extended beyond four weeks, she said.

Ardern has given her final press conference before a nationwide lockdown begins at midnight tonight to fight against coronavirus.

Ardern said as lockdown took hold, police will move around the country and ask questions of people outside of their homes. They will ask people about their activities.

People without a valid reason to be outside can be reminded of their obligations, she said.

People just getting fresh air should be able to without being told to go home by police, and some common sense was needed in their approach.

The prospect of police and the military on New Zealand streets might be daunting, but Ardern said they were there to help, and their first approach would be about asking questions and reminding people of their self-isolation requirements.

"People shouldn't be afraid of that."

Police will be the primary source of support for the community, and they had good numbers with stations nationwide.

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People will want to get some fresh air, but any common space like a surface at a playground presented risk.

When asked about whether people could go hunting or fishing, Ardern said to stay home.

"Act like you have Covid-19, it will help guide your decisions."

She said people may not be working, but they had a job and that was to stay at home.

"I know how hard this will be, especially for New Zealanders who face the first day unemployed and with an uncertain future."

She pointed to the measures the Government has made to save jobs and businesses, and to help home owners and renters.

"It won't be easy, but the alternative is worse.

"You are not alone. You will hear us and see us daily as we guide New Zealanders through this period."

She said success will not be instant, and what we do today won't see benefits for days to come. The number of cases will continue to rise, and modeling suggested that the number of cases could reach the thousands before dropping.

She said the alert level could be dropped again if the number of cases declined - though that would take some time. The nature of cases was also important, including how widespread community transmission is.

There are 50 new confirmed or probable cases in the last 24 hours, bringing the total to 205.

Community transmission is attached to cases in Marist College and from the World Hereford Cattle conference in Queenstown, while there are four other cases of community transmission.

Covid-19, left unchecked, would have an unacceptable toll.

New measures

A national state of emergency now meant the Government had all the enforcement powers it needed to fight the spread of Covid-19.

The decision was not taken lightly, and she would front to the media to answer questions and Parliament's special select committee - chaired by Simon Bridges - would allow an accountability mechanism.

Parliament would also pass an urgent bill that changed various pieces of legislation, including freezing all rent increases and empowers the Education Secretary to open and close schools as necessary.

Border enforcement would step up, she said, so that New Zealanders returning home from overseas will be screened. The number of New Zealanders coming into the country was speculative, as travel plans had changed. But Ardern said up to 10,000 could come home by the end of the month.

"If they are symptomatic, they will be tested and they will be put in isolation in an improved facility."

Asymptomatic people without the means to self-isolate will also be put in an approved facility.

They will be checked on by police in the days that follow, and they will be fined and quarantined if they are not follow instructions to self-isolate.

Approved isolation facilities will be hotels, mainly, and the Government has already looked at hotels near airports to be used.

"These are the measures we must take," Ardern said.

The new requirements would be in place at 2am today, but Ardern said there were still details being worked out, including how the Government would pay for the use of approved facilities.

She said Kiwis overseas may need to stay where they are as flights home dried up.

Germany was chartering flights to New Zealand to fly Germans home, but Ardern said New Zealanders were scattered all around the world.

"We took proactive steps to send a message to New Zealanders around when they needed to return home, and now that window is closing."

She had spoken to her Australian counterpart Scott Morrison about the 200,000 Kiwis living in Australia but are ineligible for welfare in Australia, and any New Zealander wanting to return to New Zealand would be looked after.

"Right now it's not just an issue of fairness, but an issue of public health as well." That was why she would continue to push the case for the rights of New Zealanders in Australia, she said.

The Government had supported Air NZ to keep their major international flights open for longer, but those were finishing up and Kiwis offshore needed to realise they may not be able to get home for a while.

Essential services

All non-essential services will be shut down for four weeks.

Ardern said the rules of the lockdown may need to be amended, but if in doubt, services and businesses should be closed.

"We will make changes if we have to, but now is not the time to be relaxed or flexible."

Workers in child welfare and family violence were preparing for an increase in domestic violence, Ardern said.

Police and the Defence Force were working with agencies to help people in emergency homes and homeless people - and others who may not have a safe place to self-isolate.

She said individuals should have access to services that they have always had. NGOs working in those areas were essential services, she said, but they had to use physical distancing.

Asked about great job losses in Queenstown, Ardern said the state of emergency powers allowed more flexibility in welfare payments.

'Stay home, break the chain, save lives'

A civil defence emergency was declared at 12.21pm today, giving Civil Defence director Sarah Stuart-Black extraordinary powers to enforce the nationwide lockdown.

While in force, it will allow Civil Defence Emergency Management controllers to provide for the:

• conservation and supply of food, fuel and other essential supplies.
• regulate land, water and air traffic.
• close roads and public places.
• evacuate any premises, including any public place.
• if necessary, exclude people or vehicles from any premises or place.

Stuart-Black said she hoped those powers were not necessary, but there would be zero-tolerance for people flouting the rules of self-isolation and the police could bring in the military, if necessary.

The PM said in her statement to parliament on the emergency: "At 11.59pm tonight, we move to the highest Alert Level of 4, and we, as a nation, go into self-isolation.

"The trigger: early evidence of community transmission of COVID-19 in New Zealand.

"But unlike so many other gravely inundated countries, we have a window of opportunity to stay home, break the chain of transmission, and save lives. It's that simple."

Eight people who flew in to Auckland Airport yesterday had tested positive for Covid-19, and Ardern has said she was looking at quarantining people arriving into the country.

Yesterday Ardern appeared frustrated that non-essential shops and cafes were still open, even though the alert level was only at 3 and won't move to 4 until midnight tonight.

"No bars, no restaurants. We should be in shutdown now for those services," she said yesterday.

"Obviously it takes time for us to get to a position where everything is settled into alert level 4, but I want people to apply that as if it's already arrived."

She stressed the need for people to limit close contact with others.

"Every interaction we have with someone else increases the risk of spreading the virus.
Stay at home. That is the simplest way to save lives."

People in lockdown should stick to their "bubbles" which, for most people, are those in their households.

Couples who don't live together or parents with shared custody could be in the same bubble, as could the "buddy" of a person living alone - but bubbles shouldn't overlap.

"You can't spend time with other people outside of your bubble."