Prime Minister Jacinda Ardern has announced a four-level Covid-19 alert system and asked people to stop non-essential travel within New Zealand.

Watch the Prime Minister's address to the nation:

Ardern said all people over 70 or who have compromised immunity should stay at home as much as possible and advised against any non-essential travel within New Zealand.

Addressing the nation at midday, she said she wanted to give New Zealanders "certainty and clarity" about the risks of Covid-19.

"I understand that all of this rapid change creates anxiety, and uncertainty. Especially when it means changing how we live. That's why today I am going to set out for you as clearly as possible, what you can expect as we continue to fight the virus together."

Another 13 cases were confirmed today, bringing the total number in the country to 52. Announcing the new cases, Director-General of Health Dr Ashley Bloomfield said the Ministry of Health could not rule out the risk that there was community transmission in two cases.

Ardern said the vast majority of people who got the virus would experience mild symptoms, but some would need more care - and that's why it was so important to "slow it down".

Ardern announced an alert system with four levels.

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The first was when it was here, but contained. That meant New Zealand needed to prepare - introduce border measures, contact tracing and cancel mass gatherings. These had been activated.

The second was where the disease was contained, but the risk was increasing. At this stage work needed to be done to reduce contact with each other, including further border restrictions, cancelling unnecessary travel and encouraging people to work remotely when possible.

At level three it was becoming increasingly difficult to contain. Public venues and non-essential businesses would close.

Level four meant there was sustained transmission, which meant people would need to eliminate contact with each other altogether. Essential services would still operate, but people would need to stay home if they could.

"It's important to note, that at every alert level supermarkets and essential services, like access to pharmaceuticals will continue. Shop normally. If we do that, our supermarkets will have time to restock their shelves."

Photo: RNZ

Ardern said New Zealand was currently on level two, but this could change at any time - and it could change quickly.

Level two "means the risk of community transmission is growing."

People over 70 and people with compromised immunity or underlying respiratory conditions should now stay at home as much as they could.

"That means we need friends, family and neighbours to support our older New Zealanders and people who may be in this group by doing simple things like keeping in contact and dropping off food or other supplies.

"And when you do, make sure you are not sick, that you are using good handwashing practices, and keeping your distance."

Everyone would also need to start working differently, she said.

Work from home and if you can't - take breaks at different times, she said. Not everyone would be able to work from home so those who had to stay at work should make sure they physically distance themselves from others as much as possible.

People must also limit their movement within New Zealand and cut any non-essential domestic travel to help stop the virus spreading within the country.

Schools would remain open, but be treated on a case-by-case basis, closing if there was a positive case related to it.

"Sending children home at this stage though, doesn't necessarily reduce transmission in the community, but I can assure you we are constantly monitoring these settings to keep children safe. As a mum, I can assure you that is my key consideration."

Ardern said she would share daily updates and warned alert levels could move from one to the next in a short space of time.

She said New Zealanders were practical and community-minded.

"We may not have experienced anything like this in our lifetimes, but we know how to rally and we know how to look after one another, and right now what could be more important than that," Ardern said.

"So thank you for all that you're about to do. Please be strong, be kind, and unite against Covid-19."

Read more about the Covid-19 coronavirus:

As Covid-19 spreads around the world, it can be daunting keeping up with the information. For RNZ, our responsibility is to give you verified, up to the minute, trustworthy information to help you make decisions about your lives and your health. We'll also be asking questions of officials and decision makers about how they're responding to the virus. Our aim is to keep you informed.