Prime Minister Jacinda Ardern gives an explanation of any move to level 3, but stops short of announcing a move out of level 4.

* This explainer on the Covid-19 alert level system was first published when the system was announced, and has been updated to reflect changes to the system made as New Zealand entered the fourth week of a level 4 lockdown in April.

EXPLAINER: New Zealand's system to manage the threat of coronavirus, or Covid-19, is a bit like the system we currently use to assess fire risks or terror threats.

As the threat level rises, increasingly severe measures are rolled out to combat the spread of the virus.

Those measures are restrictive and they affect many, if not every, part of our daily lives, from whether or not people can travel within the country, to how close you're allowed to sit next to people on a bus.

The threat levels range from one to four, with four being the most severe. Importantly, the threat levels can apply to the whole country or to specific regions depending on what is happening on the ground.

123.rf New Zealand now has an alert system for the coronavirus threat. The alert level will be updated daily by Government.

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Prime Minister Jacinda Ardern announces the coronavirus alert system. First published, March 22.

ESSENTIAL SHOPS WILL ALWAYS BE OPEN

No matter the threat level, essential shops like supermarkets and pharmacies will remain open but people are encouraged to shop normally so supermarkets have time to restock their shelves.

LEVEL FOUR THREAT: DON'T GO ANYWHERE

The top threat level, level four, advises people to eliminate contact with others. At this level it is likely that the disease is not contained.

The country would enter this threat level if there were a widespread outbreak, with a "sustained and intensive transmission" of the disease.

People would be instructed to stay home, and interact only with the tightly-held "bubble" of people who live with them. Schools and other educational institutions would be closed, as would businesses with the exception of those providing essential services, including supermarkets, pharmacies and clinics.

Supplies would be rationed, and certain essential facilities would be requisitioned by the Government to combat the spread of disease. Healthcare services would be heavily reprioritised and travel severely limited.

LEVEL THREE THREAT: LIMIT TRAVEL, PUBLIC VENUES CLOSED

At level three, there is a "heightened risk that disease is not contained". The country or a specific region will reach level three if there is a community transmission or multiple clusters of infection break out.

At level three, people will be asked to stay at home and will only be permitted to interact with a small "bubble" that could include close family, a caregiver, or a grandparent if appropriate precautions are taken.

The government will limit travel between regions, with exceptions for essential workers, people in shared bubbles, relocating house or business, for medical reasons, or to meet a court order.

Schools and other educational facilities will open to children up to the age of year 10. But attendance will be voluntary, and parents are encouraged to keep their children at home where possible.

Mass gatherings will be cancelled, public venues closed, and people should work from home where possible. Construction, forestry, and other businesses which can operate while adhering to physical distancing will be permitted.

Businesses will be allowed to open only if they can provide "contactless" service, so it includes deliveries or click-and-collect. Cafes, restaurants, and retail business will have to shut their doors to "face-to-face" customer contact.

Recreation will also be limited to low-risk activities such as swimming, surfing, fishing from wharfs, mountain biking, and day tramps. Coastal rock fishing, hunting, and overnight hikes are not permitted.

Doctors and nurses in primary care will not do face-to-face consultations and resources in the health system will be reprioritised due to the coronavirus threat.

SUPPLIED Prime Minister Jacinda Ardern made an address to the nation about how the government is dealing with coronavirus.

LEVEL TWO: REDUCE CONTACT, LIMIT NON-ESSENTIAL TRAVEL, WORK FROM HOME

At level two, the government advises people to reduce their contact with others. The spread of the virus is contained, but there is a growing risk of community transmission.

Threat level two is activated when one of the following is occurring: there is an increase in household transmission of the virus, or single or isolated cluster outbreaks.

Formerly, level 2 meant an extreme strengthening of the border, restrictions of mass gathering, and advice to physically distance on public transport.

Subsequent changes to the alert system mean mass gatherings of 100 people or less are permitted indoors, and gatherings 500 people or less are allowed outdoors.

People are advised to keep one metre apart, particularly on public transport. Where possible people should stay home, but participation in sports and recreational activities is acceptable.

Non-essential travel around the county should be minimised, and there are no firm restrictions on using mass transport unless a person has Covid-19 symptoms, is awaiting a test results, or is required to self-isolate.

Public venues will be opened, schools are open to all children, and hospital and medical facilities should operate as normal. Businesses, including bars and cafes, can open if they are able to comply with health and safety requirements - such as maintaining a list of customers for contact tracing purposes.

People considered to be at high-risk are advised to remain at home. This includes people over 70 or those with other medical conditions that may make them susceptible to the disease like respiratory conditions or immune deficiencies.

LEVEL ONE: KEEP OUT PANDEMIC; BORDER RESTRICTIONS

At threat level one, the spread of the virus is believed to be contained, but the government advises people to prepare for an outbreak.

David Ramos Increased quarantine measures, like those seen in Europe could be rolled out as part of a coronavirus response.

There's still a heightened risk of importing the disease or evidence of sporadic imported cases, but no widespread community transmission.

At this stage, the government will maintain severe restrictions at the border, including the quarantining of possible incoming Covid-19 cases. Also implemented will be contact tracing, self-isolation and quarantine of cases found domestically.

There will be intensive testing and physical distancing will be encouraged, but is not required. Mass gatherings are allowed, and domestic travel will not be restricted.

People will be encouraged to wash their hands, and stay home if they're sick.

CUMULATIVE MEASURES

The measures the government rolls out for each stage are cumulative. That means the measures for level two, like an effective shutdown of the border, also apply at level four.

The government cautions to take the threat levels seriously, not just for one's own health, but for the health of others.

"Think about your grandparents, think about someone in your family group or your friendship group that might have compromised immunity," Prime Minister Jacinda Ardern said when announcing the alert system in March.

"It's not about whether or not you're worried about yourself. It's that you should be worried for those around you, so please take it seriously. This is about saving lives."