Authorities in the Australian city of Melbourne have confined people living in nine housing estate tower blocks because of an outbreak of coronavirus.
The 3,000 or so residents of the blocks are being told not to leave their homes for any reason for at least five days.
At least 23 cases of infection were found on two estates in recent days.
The state of Victoria recorded 108 new cases on Saturday, its second-biggest daily increase. Australia as a whole has seen 104 coronavirus deaths.
There have been at least 8,362 infections nationwide.
Victoria State Premier Daniel Andrews said the latest figures there were a very real concern to everybody.
How different is the response in Melbourne?
The city is putting "stage three" restrictions on 12 suburbs. People there will only be able to leave the house for four reasons: work or education, exercise, medical care or care-giving, and shopping for supplies.
But the nine tower blocks in Flemington and North Melbourne face a "hard lockdown".
According to Victoria's Minister for Housing, Richard Wynn, some of the state's most vulnerable people live in the nine towers, and mental health and drug and alcohol support will be offered.
The towers, he said, were characterised by having common lifts and common walkways, and they presented an "acute challenge going forward".
Mr Andrews said that it should take at least five days to test everyone in the towers.
"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," said Deputy Chief Health Officer Annaliese van Diemen.
"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."
Acting Chief Medical Officer Paul Kelly said a "new phase of the pandemic" was under way in Australia.
"For the first time in Australia it's an actual complete lockdown," he said, as quoted by the Sydney Morning Herald.
"That will be a very difficult but important step. We know from previous pandemics… that it was a tower block [in Hong Kong] which was part of the major spread of that Sars coronavirus."
How big a setback is this for Australia?
For months the country had felt optimistic about containing Covid-19, then the resurgence of the virus in Melbourne put those efforts at a critical stage.
While the infection numbers remain below Australia's peak in March, what concerns the authorities is that local transmission is now the key source of infections.
Previously, most cases came from travellers returning from overseas.
Other Australian states dramatically slowed or eradicated the virus but in Victoria private security guards overseeing hotel quarantines reportedly broke the rules.
Mr Andrews described cases of illegal socialising between staff, listing examples of workers sharing a cigarette lighter or car-pooling. Local media also reported claims of sex between guards and quarantined travellers.
The government has ordered a judicial inquiry into the state's quarantine operation and has fired contractors.