Victoria has ramped up its action against COVID-19 as infections rise. The state recorded 127 new cases on Monday, its highest daily increase.
There are now swathes of Melbourne under lockdown so residents can only leave home for essential purposes.
Nine blocks of flats have gone into the more severe "hard lockdown", where residents can't leave their flats at all for the five days it will take to test all of them.
New South Wales and the ACT are blocking travel from Victoria.
Is this a second wave?
There is no formal definition of a second wave of infections but professor Adrian Esterman, an epidemiologist at the University of South Australia, said the current peak of cases was already higher than first and that added up to a second wave.
"Victoria is now close to getting out-of-control," he said. "It's getting to the stage where they can't contain it, in that they are getting cases outside the hotspots. We are seeing an increasing proportion of positive test results."
He thought the state should reimpose the initial severe lock-down where everybody was confined to their homes and restaurants were closed apart from for takeaways.
If it really gets out-of-control in Victoria, tracing contacts would be beyond the system - there would be too many to trace. The task would then be "containment" within the borders of Victoria - which is why New South Wales moved so quickly to close its border.
Why are blocks of flats a problem?
Blocks of flats are notorious as incubators and spreaders of infectious diseases. In the outbreak of severe acute respiratory syndrome (SARS) in 2003, Hong Kong health officials sealed off large apartment buildings.
It's not just that blocks are cramped with close face-to-face contact between people but they also have surfaces touched by many people - like lift buttons and hand rails. Lifts and stairways are narrow.
People catch COVID-19 by breathing in the virus on droplets, either exhaled by infected people or left on surfaces. People touch surfaces, pick up the droplets, touch their faces and then breathe in the virus.
Australia's acting chief medical officer, Paul Kelly, said of the Melbourne apartment blocks: "Those towers have a large concentration of people in a small area. They are vertical cruise ships in a way and so we have to take particular notice and particular attention to make sure the spread is minimised and people are protected."
Why lock down these particular blocks?
It's estimated 3000 people live in the nine locked-down towers.
Some of the blocks are around 20 storeys high. They have communal stairways, corridors, lifts and other areas. On the latest count, there had been 53 cases detected in the nine locked-down towers.
Victoria's chief health officer Brett Sutton said on Monday: "It's an increase of 26, essentially doubling of the numbers from yesterday (Sunday).
"It is exactly the reason why these towers are in a hard lockdown and why we're doing extensive testing across all of them."
There may be more cases which have yet to be detected, according to Dr Annaliese van Diemen, Victoria's deputy chief health officer (communicable disease).
"We are extremely concerned that there are many hundreds of people in these towers who have already been exposed to the cases that we've found and possibly to cases that exist and that we haven't found," she said.
"This is not just a matter of 23 to 30-odd people. This is a matter of many hundreds who have already been exposed and who may already be incubating."
One of the problems may have been a lack of English and the authorities have been criticised for not recognising that.
Professor Esterman said some of the residents came from brutal dictatorships and so they mistrusted authority.
Why lock down flats with no cases?
The flats are not occupied only by refugees, though some of the residents were fugitives from terrible situations.
Refugees, many of whom are without doubt intelligent and respectable, are in a new, strange country without much money. They may mix socially with similar people from their original part of the world. The community is across the blocks and not within one block.
The authorities are not blaming the residents. "It's not about the people who are there, it's about the entire environment and the way that people interact and the issue of how easily this virus spreads," professor Sutton said.
Now it's political
Unlike in other countries, particularly the United States and United Kingdom, Australia has largely kept politics and plague apart.
But in Victoria, there is criticism of Premier Daniel Andrews' Labor government because of the way the infections spread from hotels for people returning from abroad.
It turned out security guards employed by a private firm were the carriers. Local media reported guards shared cigarette lighters and cars.
They are also alleged to have had sex with people in quarantine.
Mr Daniels ordered an enquiry but this hasn't completely deflected the flak.
Criticism has come from the right. "Daniel Andrews is now clearly the worst-performing, most unsuccessful premier or territory leader in managing COVID-19," Greg Sheridan of The Australian wrote.
And from the left. "There was no training provided to these security guards which were placed in these hotels which had very highly-infectious diseases," said Kazim Shah, of the United Workers Union.