MELBOURNE - More than five million residents of Melbourne will be locked down for six weeks after coronavirus cases surged in Australia's second-biggest city, authorities announced Tuesday.

State Premier Daniel Andrews said the lockdown would begin at midnight Wednesday and last at least six weeks, as he warned residents "we can't pretend" the coronavirus crisis is over.

After the south-eastern city detected 191 new cases in 24 hours, Andrews said there were now too many incidents of the virus to trace and track.

"These are unsustainably high numbers," he said. "No-one wanted to be in this position. I know there will be enormous amounts of damage that will be done because of this. It will be very challenging."

Most school students will return to remote learning while restaurants and cafes will be limited to serving takeaway food.

"There is simply no alternative other than thousands and thousands of cases and potentially more," he told reporters.

Although the lockdown covers the Melbourne metropolitan area, the entire state of Victoria will effectively be sealed off from the rest of the country from Tuesday midnight, as state borders are closed.

Police and the military are patrolling dozens of border crossings and using drones and other aircraft to surveil the vast frontier with other states where the coronavirus has been successfully contained.

Health officials last week effectively shut off some 300,000 Melbourne residents to the rest of the city until the end of July, but that has now been extended beyond their neighbourhoods.

Roughly 3,000 people were also locked in their homes on Saturday in Australia's strictest COVID-19 response to date after a cluster emerged in a high-rise public housing estate.

So far, 69 cases have been recorded across the nine densely populated towers and there are concerns the virus could spread widely, with one health official likening the crowded conditions inside to "vertical cruise ships".

Cruise ships emerged as early coronavirus hotspots, with passengers and crew often packed in small cabins and at high risk of infection.

Australia has recorded almost 9,000 cases of COVID-19 and 106 deaths from the virus.

Almost all new daily cases are being detected in Melbourne, while all other regions are enjoying relaxed restrictions after largely curbing the virus spread.

AFP