The declaration gives the federal government power to intervene and deploy resources.
It is a legislative power that Morrison sought after the 2019-2020 bushfire crisis.
Morrison is also set to outline more money to help communities recover from floods on the east coast when he visits Lismore today, as government ministers expect anger from residents whose lives have been upturned.
The federal government has been criticised for the pace and scale of its response to the floods.
The amount of funding is not yet known but was approved by the federal government's expenditure review committee yesterday and is being described as significant.
Senior government figures including the Treasurer have already estimated recovery costs in the billions of dollars.
Shadow Minister for Disaster and Emergency Funding Murray Watt said federal funding for flood recovery needed to be generous and reflect the scale of destruction.
"If he turns up with a couple of hundred million dollars or something like that, it's just not going to do the job," Watt said.
Earlier today, Watt backed a national emergency declaration, saying the NSW state government had been overwhelmed by the disaster.
However the NSW government has not yet declared a state of emergency - NSW Premier Dominic Perrottet said yesterday it was deemed unnecessary to do so.
"The advice we have received is that is not necessary at this stage and it is something we discussed on a daily basis," Perrottet said.
"We believe that the coordination that we have in place between the Commonwealth and the state and all of our agencies has the capacity to deal with the situation in front of us."
NSW SES commissioner Carlene York agreed yesterday that there was no need to declare a state of emergency at that stage.
But the Prime Minister's Office insisted the national emergency declaration was necessary to ensure nothing would stand in the way of flood assistance.
National emergency trigger has never been pulled
The prime minister's national emergency declaration will allow the federal government to deploy Defence Forces without waiting for a request from state governments for assistance.
The idea of declaring a national emergency emerged out of the royal commission into natural disasters, called in the wake of the Black Summer bushfires.
It suggested a national emergency would preferably be called early in a disaster, signalling to communities the seriousness of the incident and putting government agencies and defence forces on high alert.
The royal commission suggested the declaration "should be the catalyst for a more 'coherent, pre-emptive and expeditious' mobilisation of Australian government resources".
Though the federal government had the power to intervene earlier than today, Agriculture Minister David Littleproud said NSW should have made the request for help sooner.
"[Defence Minister] Peter Dutton had the defence force personnel ready - and any asset that they wanted was ready at their disposal," Littleproud said.
"This is a simple solution - this is about the states simply saying 'bring them in' sooner - and I think that's the lesson that the NSW government and other state governments need to learn and understand."
Deputy Prime Minister Barnaby Joyce, who also plans to visit the region tomorrow, said he expected people would be angry at the government.
"I am fully aware of what I am about to walk into, and the prime minister, and other people," Joyce said.
"You are not going to meet happy people. You are going to meet people whose lives have been turned upside down.
"They want to be heard and that's the main thing you do … you try your best to listen to them."
Photo: AFP / Australian Defence Force / Dustin Anderson Caption: Federal and state governments have been criticised for a slow response to the flooding disaster - here soldiers load food for delivery to areas of northern New South Wales affected by floods.