Australians are being ordered against travelling overseas as a blanket ban on indoor gatherings of more than 100 people was announced under new coronavirus measures this morning.

After declaring a human biosecurity emergency, Prime Minister Scott Morrison said states and territories would enforce the tough new restrictions with immediate effect and it could last for at least six months.

Mr Morrison said the government is determined to stop the spread of COVID-19 and save lives.

Prime Minister Scott Morrison and Chief Medical Officer Brendan Murphy. (AAP)

"This is a once in 100-year type event," Mr Morrison said, in a landmark press conference during an outbreak which has so far infected 454 people in Australia.

International travel 'ban'

For the first time in Australia's history, the prime minister upgraded a "travel ban" on Australians to Level Four for the entire world.

"The travel advice to every Australian is do not travel abroad," Mr Morrison said.

Although the prime minister described the Level Four alert as a "ban" on Australians going overseas, it's not strictly a ban.

But people are being strongly discouraged from travelling overseas at all.

(Supplied)

Restrictions on gatherings

There is an immediate ban on non-essential gatherings of greater than 100 people in indoor areas. For outdoor areas, the limit remains at 500.

The restrictions on indoor gatherings will not apply to workplaces, public transport, supermarkets, shops and shopping malls. Correctional facilities, youth justice centres and courts are also free from the bans. Churches, mosques and other religious bodies must follow the ban.

There will be severe economic implications for the travel, tourism and aviation sectors, which will spread more broadly across the economy, the prime minister warned.

"Tens of thousands of jobs could be lost, if not more," he said.

Pubs, restaurants and cafes and entertainment are expected to be heavily affected.

Parents and schools are preparing for the increasing likelihood of a school closure. (Janie Barrett)

Schools to remain open – for now

Despite growing concerns around the country, Mr Morrison was very clear that schools would remain open.

"I am telling you that, as a father, I'm happy for my kids to go to school. There is only one reason your kids shouldn't be going to school and that is if they are unwell," he said.

"The health advice here, supported by all the premiers, all the chief ministers and my government is that schools should remain open."

Standing alongside Mr Morrison, Australia's Chief Medical Officer Professor Brendan Murphy acknowledged school closures were a "controversial issue".

"We believe very strongly that it's in the best interest of our children and the nation at this time to keep schools open," he said.

If a big outbreak in a community occurs some local school closures might be necessary, Prof. Murphy said.

A woman wears a protective mask as COVID-19 outbreak spreads around Australia and the world. (Nine)

Biosecurity Act and no lockdown

Mr Morrison said Australians should "not be alarmed" that a human biosecurity emergency was declared under the Biosecurity Act by the Governor-General.

The national security committee met yesterday before the national cabinet and the act was triggered this morning.

Mr Morrison said the action recognised the serious threat of the coronavirus.

But Australia was not going down the path of shutting down society like some other countries have.

Prof. Murphy said a lockdown of just a month or two would not work, as the virus would return.

"We have to have sustainable measures, but they have to be serious measures," he said.

"There is no way that we can lock down society and make everyone stay home and then in a month's time, undo that, because the virus will just flare up again without any real long-term benefit."

Aged-care and visit limitations

Some visitors will not be permitted to enter aged care facilities in new restrictions that will affect the elderly.

People with symptoms and anyone not vaccinated against influenza will be told to stay away, and visits will be limited to a short duration to visitors at a time per day.

However, Mr Morrison said aged care facilities have the discretion to put in strict arrangements to allow people to visit their loved ones in certain "compassionate" circumstances.

Anzac Day events and ceremonies are to be cancelled.

Mr Morrison confirmed his government had lifted working restrictions on 20,000 student nurses, who can now join the frontlines.

Empty toilet paper shelves in a Coles store in Sydney. Some customers have stockpiled toilet rolls and other essential items. (AAP)

Supermarket hoarders scolded

Mr Morrison hit out strongly at Australians who had stripped supermarket shelves of goods and had been stockpiling items in their home.

"Stop hoarding. I can't be more blunt about it," he said.

"It is not sensible, it is not helpful and it has been one of the most disappointing things I have seen in Australian behaviour in response to this crisis."

There was no reason for people to be hoarding supplies in fear of a lockdown, he said.

"We are going to keep Australia running. We are going to keep Australia functioning."

Financial boost for airlines

The government hopes a waiver of aviation-related charges, beginning in April, will help commercial airlines in Australia.

Mr Morrison did not specify details about the fees, but he did confirm there would be rebates on any payments dating back to February 1.

"This will provide support for our aviation sector who has been the hardest hit ... as you have seen from the various announcements from the major airlines."

Smaller regional airlines would not be forgotten, Mr Morrison said.

"They don't necessarily change the world for those airlines but they do provide some support at a very difficult time."