Most children will not be allowed back to school after they left today - possibly for many months.

Schools and early childhood services learnt through the media this afternoon that they cannot accept any students from tomorrow except children of parents who work in a list of "essential services" which has not yet been issued.

Epsom Girls' Grammar School told parents at 2.45pm that the school would close from 3.15pm today for four weeks, except for the children of essential workers.

Principals' Federation president Perry Rush said there was no communication about the decision to schools before 3pm today, when most schools start to close for the day.

"No, I think other than the comment in the media, that's all we have at this point," he said.

"We are yet to take that from the Secretary [of Education], so it will be principals needing to communicate that to their communities electronically at some point this afternoon, I would think."

Prime Minister Jacinda Ardern announced at about 2pm that schools and early childhood centres would close "from tomorrow" except for the children of essential workers.

A spokeswoman for Education Minister Chris Hipkins confirmed that that meant from the end of today, not from the end of Tuesday.

Pt Chevalier School principal Stephen Lethbridge said he watched Ardern's press conference like anyone else.

"Leadership teams from around the country were around computer screens and as the announcements were being made our team was busy writing on glass walls the things that we needed to put in place and what needed to happen," he said.

"We talked about it with the kids in our classes - that hey, look, it's been announced by the Government that school is going to close tomorrow and we'll probably have a big holiday, so it's really important that we all stay home and follow all the advice, but not to worry because being at home is the safest place to be," he said.

"I'm yet to communicate to the community, but that is my next job."

Hipkins told journalists at Parliament that the list of essential workforces would be provided to schools.

"Schools will communicate that to parents so parents know who can and can't send their children to school tomorrow and Wednesday," he said.

"We are asking for goodwill to continue to supply support to families that need that over the next 48 hours while they make other arrangements."

Ardern told journalists that the list of "essential services" would include healthcare, the food supply chain from farmers to supermarkets, police, defence, Ministry of Social Development staff who handle welfare benefits, media workers and certain construction workers.

"We will have essential services for building and construction, for example if a water main breaks at a hospital or we are completing building works at a hospital," she said.

Even these workers are expected to make other arrangements for their children by the end of Wednesday, when all schools and early childhood centres will be "closed entirely".

Hipkins said tertiary institutions would also close "as soon as possible".

"Funding for early learning and tertiary providers will not be cut or clawed back based on non-attendance or failure to meet key performance indicators," he said.

"All schools and early learning services will be closed from midnight on Wednesday, although distance learning may continue to be provided where it can be done consistent with self-isolation."

The closures apply to the next four and a half weeks until the end of the Easter school holidays on April 28, but look likely to be extended, potentially for many months, until the coronavirus pandemic is contained.

Hipkins said the Government was "working very hard to scale up online learning opportunities" for children while they are stuck at home.

He said there were "equity concerns" for about 20 per cent of children because they did not have suitable computer devices or broadband at home.

"We are making sure we have got broadband supplied in households where there are kids at home. That is something we are working on very closely with the telecommunications companies," he said.

"Then there is the issue of devices and we will be working hard to make sure that as much as possible we overcome that difficulty."

Rush said he was confident that schools would cope despite the short notice.

"I think schools have been aware, of course, that this could be one of the options that was placed in front of the Cabinet," he said. "We have certainly known for at least two weeks that this could happen."