Government officials have warned Australians not to get complacent about coronavirus just because it's Easter.

Amidst concerns that people may not be able to resist bending the rules to catch up with family and friends, leaders have said measures could be further tightened if infections spike or too many people flout the rules.

In Victoria, Premier Daniel Andrews has suggested this may see the state move into "stage 4" restrictions.

So what does 'stage 4' actually mean?

Well, the Federal Government hasn't publicly issued a breakdown of what happens in each stage of restrictions.

It does have an emergency response plan for the coronavirus that broadly outlines which bodies are responsible for issuing public health measures and what needs to happen before Australia moves to "targeted action" and "standdown" stages.

But there are no specifics that set out who has to self-isolate, for how long, what people are allowed outside for, or how long quarantine should last.

This would be difficult for the Federal Government to do because, according to the emergency plan, state governments are at liberty to enforce rules at different times depending on how COVID-19 is spreading in their state.

But talking about "stages" is the language many Australian government officials (both state and federal) have been using to describe what's happening as restrictions are scaled up in response to the virus.

It's also the way some international governments are talking about their response to the pandemic, and is a handy way to compare what's happening in Australia to how things work overseas.

So where are we at now?

As of Easter Saturday, April 11, the rules in place at a federal level are:

No indoor or outdoor gatherings of more than two people unless you're all part of the same household

unless you're all part of the same household People should not leave their house except for essential reasons: to shop for food or other essentials, attend work or education (if you can't do this from home), exercise, or for medical care or compassionate needs

to shop for food or other essentials, attend work or education (if you can't do this from home), exercise, or for medical care or compassionate needs Some state borders are closed except for people with exemptions for essential travel

except for people with exemptions for essential travel There can be a maximum of one visitor (not a member of your household) to your home — but some states are taking a less strict approach to this rule

— but some states are taking a less strict approach to this rule People must maintain a 1.5-metre distance from each other if they are in contact with members outside their household (ie. going to the shops, the workplace, out for exercise)

if they are in contact with members outside their household (ie. going to the shops, the workplace, out for exercise) Many non-essential venues are now closed to the public . Restaurants and cafes are only allowed to offer take away or home delivery, while pubs, gyms, cinemas, casinos and places of worship are closed. Some non-essential retailers are still allowed to be open as long as they comply with social distancing measures

. Restaurants and cafes are only allowed to offer take away or home delivery, while pubs, gyms, cinemas, casinos and places of worship are closed. Some non-essential retailers are still allowed to be open as long as they comply with social distancing measures All travellers arriving in Australia must go into 14 days of self-isolation at a designated quarantine facility

These rules were updated at a national level on March 30, the third time the Prime Minister introduced new measures to deal with the spread of coronavirus. That's why many are calling this "stage three".

Since Australia introduced the latest round of restrictions, which in some states include fines for people leaving their house for non-essential reasons, the rate of new COVID-19 infections has declined.

But the Victorian Premier, among others, warned on Tuesday that these early successes were not a reason to get complacent.

"If people go about their business, and have a normal Easter, then all of our hard-won gains will just be frittered away," he said.

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Mr Andrews would not rule out pushing for a new stage of measures for Victoria, if the rate of infection begins to rise again.

Elsewhere, Premier Peter Gutwein has already put the north-west of Tasmania into an "effective lockdown", to deal with a rash of cases linked to three hospitals in the region.

And their Commonwealth counterparts have similarly warned that federal restrictions will be stepped up if people don't take things seriously.

So what would new restrictions, or "stage 4", look like?

Let's look at New Zealand's stage 4

In New Zealand, Prime Minister Jacinda Ardern outlined a four-stage system, including the measures to be taken at each stage and what circumstances would trigger an escalation.

New Zealand went into stage four of these restrictions two weeks ago.

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Stage 4 is activated when it is determined likely that COVID-19 is not contained. Measures include instructing people to stay at home, closing educational facilities and all non-essential businesses, rationing supplies and requisitioning facilities, limiting travel, and a major reprioritisation of healthcare services.

Announcing the move to stage 4, Ms Ardern warned:

"If in doubt, don't go out. These measures will be in place for four weeks at this point."

New Zealand Prime Minister Jacinda Ardern holds up a card showing the series of stages on the country's alert system. ( AP: Nick Perry )

There is no universal playbook for governments during the pandemic. An Australian stage 4 could look very different to the policies New Zealand has introduced.

But the country's response offers the clearest example of what new restrictions could look like.

New Zealand has described stage 4 as an attempt to "eliminate" the virus in the community after the outbreak was deemed unlikely to be contained and widespread community transmission was detected.

On March 25, New Zealanders received a text message telling them they would have to stay wherever they spent that night for the duration of stage 4 measures and to only have physical contact with those they were living with.

Schools have been completely shut down, even for children of essential workers, whereas those in all states and territories in Australia remain open to at least these families.

All non-essential places of business have also been closed, including non-essential retailers.

New Zealand is over halfway through its stage 4 lockdown, although the Government has warned the period could be extended.

What stage 4 could mean for vulnerable people

The UK declared a lockdown on March 24, with Britons only allowed to leave their homes once a day for exercise, to shop for essential items like food, for any medical needs or to help a vulnerable person.

However, over 70s and vulnerable people have been asked to remain indoors for a full 12 weeks while the country grapples with the COVID-19 pandemic.

In the UK, there have been instructions for vulnerable people to not leave their houses for at least 12 weeks. ( Reuters: Andrew Couldridge )

Vulnerable Britons, including those going through cancer treatment or who live with medical conditions that can supress the immune system, have been told they shouldn't even leave the house to get groceries or to exercise.

Australia could introduce similar measures to protect people who are more likely to die from COVID-19.

The groups already identified by the Australian Government as being at risk during the pandemic include:

People aged over 70 and people over 65 who have pre-existing medical conditions

People aged over 70 and people over 65 who have pre-existing medical conditions People with weakened immune systems

People with weakened immune systems People with diagnosed chronic medical conditions (such as high blood pressure, heart and lung conditions, kidney disease and diabetes)

People with diagnosed chronic medical conditions (such as high blood pressure, heart and lung conditions, kidney disease and diabetes) Aboriginal and Torres Strait Islander people, who have higher rates of chronic illness

These people have already been strongly advised not to leave the home "unless absolutely necessary" but a stage 4 lockdown could see firmer measures taken to protect the most vulnerable Australians.

So how likely is it Australia will go to stage 4?

Without a crystal ball, it's hard to say.

But Australia's Deputy Chief Medical Officer Paul Kelly said on Good Friday that we could be "on the cusp" of slowing the infections to the point that the epidemic "dies out" here.

This projection is based on a magic number called the reproductive rate (also called "R0" or "R nought"), which refers to how many people are infected by one person with COVID-19.

The WHO initially estimated the R0 for COVID-19 to be between 1.4 and 2.5, but it varies from place to place depending on public health measures.

Professor Kelly said as measures like social distancing, self-isolation and quarantine come into play, we start to look at the effective R rate, based on Australia's cases.

"Ideally, where you want to be is below one, so less than one other person being infected after a person themselves had the infection," he said.

"And once you get to that point, the virus dies out, or the epidemic dies out. And so at the moment we're probably on the cusp of that in Australia."

Professor Kelly said that number has been quite high at different points during the outbreak, and has been as low as between one and two.

He also says we're in a much better place than we were a few weeks ago in terms of the daily numbers of new cases, referred to as the growth factor.

Those sound like good signs. So will restrictions be relaxed?

These are good signs, but even once those numbers go all the way down, we still don't know if people can develop an immunity or whether a vaccine will be developed. That's important to work out before we start easing restrictions.

"In terms of the virus dying out, as it were, in certain parts of Australia, that would be a great achievement," Professor Kelly said.

"It does bring with it a challenge. It would mean most of us would not have been exposed yet, and so we would remain susceptible to the virus if it was to be reintroduced to that area."

According to the emergency response plan, the Australian Government will "coordinate the stand down of enhanced measures" and work with state and territory governments to "determine when to cease or reduce measures".

Prime Minister Scott Morrison has indicated that some of these measures are likely to be in place for six months (from the end of March), at least in some form, and Health Minister Greg Hunt said on Thursday this was still the expectation.

Though he did say if things improve, the Government would look to take some safe early steps to ease the restrictions.

The key message here: stay home

The moral of the story is we don't really know if Australia will reach another stage of restrictions that force us all to stay inside, but our health chiefs and government officials are all saying the same thing.

Even though we are flattening the curve, Australians have been urged to take heed of the current advice in order to keep the number of new infections each day trending downwards.

Otherwise, governments may be pushed to ramp up their enforcement.