Victoria has declared an "unprecedented" state of emergency to respond to the coronavirus outbreak.

The declaration, which came into effect at 12:00pm on Monday March 16, gives the chief health officer Brett Sutton wide-ranging powers.

At the direction of Professor Sutton, health officials can detain people, search premises without a warrant and force people or areas into lockdown if it is considered necessary to protect public health.

This story was last updated on Saturday, March 21. Stay up-to-date with the latest news on the ABC website.

Why was the state of emergency declared?

Basically, all of these public health measures are to "flatten the curve" of the COVID-19 pandemic.

Governments and health officials are trying to slow the spread of infection to reduce pressure on the health system and hopefully reduce the number of people killed by the virus.

A key part of that is "social distancing", which is why we've seen announcements about large gatherings being banned and a mandatory 14-day self-isolation period for people returning to Australia from overseas.

Broadly, states of emergency give the chief health officer — a position currently occupied by Professor Sutton — powers to quarantine people and impose restrictions in parts of the state.

This declaration was initially made to give Victorian authorities the power to enforce two key national decisions: prohibiting non-essential mass gatherings and enforcing self-quarantine following overseas travel.

"Expanding the powers of the chief health officer will allow us to better protect public health and give our hospitals, GPs and other industries the best fighting chance we can," Health Minister Jenny Mikakos said.

The measures are designed to keep the pressure off hospitals and medical services. ( ABC Melbourne: Kristian Silva )

Which gatherings can go ahead?

At the time the declaration was made, a ban on non-essential outdoor gatherings of 500 or more people was in place.

Since then, a further ban on non-essential indoor gatherings of 100 people or more has been introduced, with conditions on the space required for smaller indoor gatherings.

But there are some key exemptions.

Even if there are more than 500 people there, you're still allowed at airports, hotels and motels, on public transport, at train stations or tram and bus stops and at medical or health service facilities.

Schools and universities are also exempt for now, but Premier Daniel Andrews has repeatedly flagged that they could be closed in the future.

While not mandated, all of the state's major universities are preparing to teach classes online in anticipation of potential campus closures.

Transiting through Federation Square is allowed, even if more than 500 people do it at once. ( Supplied: Matt Smith )

Food markets, grocery and retail stores, shopping centres, office buildings, construction sites, factories, prisons and correctional facilities are allowed to retain their normal operations.

Victoria's declaration also gives the green light to any gathering for the purposes of emergency services or in areas where people are transiting through — like Federation Square or the Bourke Street Mall.

The state's courts are also exempt, but have begun testing their ability to run hearings online, and have triaged and postponed a host of hearings in several courts.

The chief health officer can grant exemptions to operators with social distancing policies that he's approved.

Those exclusions will be reviewed on a day-to-day basis and are expected to change.

People who are worried they fall into a "grey area" with their event or gathering are urged to call the state's coronavirus hotline on 1800 675 398.

"It's impossible to draft an order that doesn't necessarily pose some questions, and the individual circumstances of every event will ultimately dictate whether they go ahead," Mr Andrews said.

What are the rules about overseas travel?

Under the declaration, anyone who arrives at a Victorian airport on a flight that originated outside Australia or on a connecting flight from a flight that originated outside of Australia must go straight from the airport "to a premises that is suitable for the person to reside in for a period of 14 days".

"If you're a returning traveller, you need to be at home or in a hotel for 14 days. That's the only safe, smart and legal thing to do," Mr Andrews said.

Anyone arriving in Victoria from another country has been told to go straight to somewhere they can self-isolate. ( Melbourne Airport, file photo )

Once again, there are some exemptions.

Those in self-isolation can only leave to get medical care or medical supplies, for any other emergency situation or "in circumstances where it is possible to avoid close contact with other persons".

Professor Sutton told ABC Radio Melbourne that could include activities like walking the dog to a quiet park, as long as you're able to keep the recommended 1.5 metres away from other people.

Other people are only allowed in and out of that accommodation during the self-isolation period if they already live there or there's a medical or other emergency.

Members of flight crews don't have to self-isolate, and neither do permanent residents or citizens of Pacific Island nations, or those who live in Pacific nations who are transiting through Victoria to get home.

What happens if people don't comply?

Under the declaration, people can be fined up to $19,826 if they don't comply with these orders and body corporates can be fined up to a whopping $99,132.

People can't be found guilty if they have a "reasonable excuse" for failing to comply.

The Premier said people had so far been following advice and "doing the right thing", so he didn't anticipate authorities would have to fine people.

The chief health officer has formally requested the assistance of Victoria Police, which gives them the power to detain and arrest people who don't comply with orders.

As a result, police have been checking up on people who should be undertaking a period of self-isolation.

What else can happen under a state of emergency?

Victoria's chief health officer Brett Sutton is given expanded powers under the declaration. ( ABC News )

Professor Sutton said while this declaration just covered public gatherings and the self-quarantine after travel, "it can apply more broadly and it can apply to much more specific circumstances".

Under the Public Health and Wellbeing Act 2008, the chief health officer or appointed "authorised officers" can have their powers expanded in four key ways during a state of emergency:

Any person or group in a particular area can be detained to eliminate or reduce a serious risk to public health

Any person or group in a particular area can be detained to eliminate or reduce a serious risk to public health The movement of people within the emergency area can be restricted

The movement of people within the emergency area can be restricted People can be prevented from entering an emergency area

People can be prevented from entering an emergency area Authorised officers can make any direction considered reasonably necessary to protect public health

The "emergency area" could be defined by the chief health officer and could contain a street, suburb or the state.

The state of emergency could see movement restricted within a designated "emergency area". ( ABC Melbourne: Simon Leo Brown )

Authorised officers are appointed by the secretary of the Department of Health and Human Services (DHHS), who right now is Kym Peake.

The officers must briefly explain why it's necessary to detain a person. If this explanation is "not practicable", the officer must do so "as soon as is practicable".

The chief health officer could also give authorised officers the power to exercise the following "public health risk powers" if it is deemed necessary to investigate, eliminate or reduce the risk to public health:

Close any premises

Close any premises Direct people and groups to enter, not enter, remain at or leave any premises. The authorised officers must give a period of time, of under four hours, for the direction to be complied with

Direct people and groups to enter, not enter, remain at or leave any premises. The authorised officers must give a period of time, of under four hours, for the direction to be complied with Enter any premises without a warrant to search and seize "any thing that is necessary for the purpose of investigating, eliminating or reducing the risk to public health". The officers can only use this power if they "reasonably" believe there may be an immediate risk to public health

Enter any premises without a warrant to search and seize "any thing that is necessary for the purpose of investigating, eliminating or reducing the risk to public health". The officers can only use this power if they "reasonably" believe there may be an immediate risk to public health Require "any information" needed

Require "any information" needed Require people to provide their name and address, inspect any premises, require the cleaning or disinfection of any premises "where the risk to public health may arise", require the destruction or disposal "of any thing"

Require people to provide their name and address, inspect any premises, require the cleaning or disinfection of any premises "where the risk to public health may arise", require the destruction or disposal "of any thing" Direct any other person to take any other action the authorised officer considers necessary

Before exercising any of those powers, the authorised officers must warn the person that failing to comply without a reasonable excuse is an offence.

More severe social-distancing measures could be implemented under the state of emergency. ( ABC News: Ron Ekkel )

If you suffer a loss as a result of directions given under the state of emergency and you think there were insufficient grounds, there is a section under the act that allows you to apply to the Secretary of DHHS for compensation.

How long will the state of emergency last?

The state of emergency will be in place for at least four weeks, and probably longer.

"It is likely that state of emergency will be extended and will be in place for as long as it needs to be," Mr Andrews said.

"The other point to make is that state of emergency, the provisions, the orders that are made as part of that will almost certainly change. We will try to minimise those changes, so that we can have as much consistency, as much certainty for people impacted."

Under the Act, the total period the declaration can continue "in force" can't be any longer than six months.

What other emergency powers could be enforced?

Mr Andrews has also flagged the possibility of a state of disaster — like the one imposed during the summer's horrific bushfire season — being declared.

While they sound similar, they fall under different legislation and are implemented through different state mechanisms.

Mr Andrews said a state of disaster could give "a broader suite of powers" to control the delivery of services, including public transport.

If they were enacted, those different powers wouldn't cancel each other out, but would instead likely be used together.

DHHS has a hotline for public information on COVID-19 which is 1800 675 398.

More information is available on the DHHS website.