Alert level 3 means the risk of the potentially deadly virus not being contained and there will either be community transmission of the virus or multiple clusters breaking out.
Level 4 means people are instructed to stay at home, schools and universities closed, as well as non-essential businesses, major reprioritisation of health services, and severely limited travel.
Essential services will be open at all alert levels, but level level 3 means limited travel in areas with clusters of Covid-19 cases, affected educational facilities closed, mass gatherings cancelled, public venues closed (such as libraries, museums, cinemas, food courts, gyms, pools, amusement parks), some non-essential businesses closed, and non face-to-face primary care consultations, with non-elective services and procedures in hospitals deferred.
Prime Minister Jacinda Ardern has just told the nation "we are all now preparing as a nation to go into self-isolation in the same way we have seen other countries do. Staying at home is essential".
That would give the health system a chance to cope, she said.
Workplaces now had to change, while essential services had to ensure a 2m physical distance between people. Schools would close from tomorrow, except for those people who work in essential services with kids at schools.
All schools would close from Wednesday, she said.
All New Zealanders not in essential services are now being asked to stay at home, but leaving the house for exercise or for a walk was still allowed.
"It must be solitary," she said, and being outside meant to be keep a 2m distance from others.
Public venues will be closed under level 3, with alternative ways of working to be found and some non-essential businesses should close.
There will be no face-to-face primary care consultations and elective surgeries and procedures deferred and healthcare staff reprioritised.
Health services, emergency services, utilities and goods transport, and other essential services are expected to remain up and running at all stages.
People will still be able to go to the supermarket, fill their car at fuel stations and collect medicine from pharmacies.
"Alert level 3 is where the disease is increasingly difficult to contain," Ardern said on Saturday.
"This is where we restrict our contact by stepping things up again. We close public venues and ask non-essential businesses to close."
The chance of widespread community outbreak was expected to remain low, according to the Ministry of Health's website.
The Prime Minister today said that, when the country moves to alert level four, contact tracing would continue and testing would go on "at pace" to find out where cases are.
"If we flush out cases we already have, and slow down transmission, areas could move out of level 4," she said.
Community transmission had a lag time, and these measures would be in place for at least four weeks, she said.
She repeated that pharmacy products will still be available, and supermarkets would stay open.
Seeing a family member for lunch or hanging out with friends would risk keeping New Zealand at level 4 for longer, she said.
"I do not underestimate what I am asking New Zealanders to do. It is huge. And I know it will feel daunting."
Without these measures, up to tens of thousands of people could die, according to medical modelling considered by Cabinet today.
"The worst-case scenario is simply intolerable," she said.
It would be the greatest loss of life to a single event in New Zealand's history.
"I hope that you are all with me on that decision," the Prime Minister said today.
She the Government had done all it could to prevent the spread of Covid-19, and now the Government was asking the New Zealand public to do the same.
The measures would hit the economy hard, but they were necessary, she said.
