Prime Minister Jacinda Ardern says she has one simple message for New Zealanders on the eve of an unprecedented lockdown - 'stay home'.

"It will break the chain of transmission and it will save lives," she said.

"You are not alone, you will hear us and see us daily, as we guide New Zealand through this period. It won't always be perfect, but the principle of what we're doing is the right one."

Watch the PM's latest media conference here:

Ardern has spoken after the country entered a State of Emergency at 12.21pm this afternoon.

Ardern said breaking the rules could risk killing someone close to you and could contribute to extension of the lockdown, due to begin at 11.59pm tonight.

Police would be monitoring people and asking questions of people who were out and about during the lockdown to check what they were doing.

Urgent business has been dealt with in Parliament as authorities establish lock-down systems.

The Prime Minister said the government now had all of the legislative and other tools it needed to combat the spread of the coronavirus, and Civil Defence controllers now had power to take the actions they needed to to protect the country.

Ardern said the government was doing more work with the scientific community to move down the alert scale, but that required a reduction in cases of Covid-19.

Ardern said essential services rules may be changed but said if in doubt, do not open your business.

She acknowledged that some were going into the lockdown unemployed, but she was aiming to reassure them the government would support them during the lockdown.

"As we enter into a stage that none of us have experienced before, I want to share a few messages. You are not alone... Success won't be instant... You may not be at work but that does not mean you don't have a job. Your job is to save lives.

"If you have any questions and you are looking for answers, apply a simple rule - act like you have Covid-19.

"Be kind, stay at home, break the chain."

Watch the PM's interview with Checkpoint here:

Ardern also announced today that border restrictions would be ramped up and everybody coming into the country would be screened. She told Checkpoint anyone who showed symptoms or did not have a self-isolation plan would be taken to approved facilities.

"By approved facilities we essentially mean a quarantine, so they'll be going into hotels or for instance the facility we used at Whangaparaoa. They'll be there for 14 days. So that's anyone who's symptomatic, anyone who doesn't have a self-isolation plan or the ability to get to self isolation without coming in contact with others.

"We are also putting additional expectations on those who say they have all those plans in place and can get there without contacting or contact with others. Our expectation is police will follow up on those individuals. If they are not in self isolation, they will be being quarantined and fined.

"We have as many as, this is an estimate, as 10,000 [isolation places available] through to roughly the end of the month, but I'd say that it could be on the high side.

"We have several thousand places so sufficient to meet our needs for those coming through the border."

Earlier today, MPs passed the bill which put in place the necessary arrangements in order to implement Covid-19 Alert Level 4 that starts at 11.59pm tonight.

It also introduces measures such as rent freezes and protecting against evictions during the lockdown period.

MPs have also passed legislation allowing the government access to $52 billion for the lockdown and various support measures.

Legislation has also been passed approving urgent tax and social assistance changes, including raising the provisional tax payment threshold, and removing the hours test for the In Work Tax Credit.

Civil Defence Emergency Management director general Sarah Stuart-Black said a state of national emergency was declared at 12.21pm today.

The State of Emergency legislation allows it to be in place for seven days and can be extended.