SYDNEY (REUTERS) - Australian authorities conducted mass tests on Tuesday (Nov 17) and about 4,000 people were confined to quarantine in the hope of stifling a new cluster of cases of the novel coronavirus after hopes it had been largely eradicated.

The state of South Australia on Monday reimposed social distancing restrictions after detecting 21 cases of Covid-19, the bulk of which were acquired locally.

The cases were the first local transmission of the virus in Australia in nine days.

South Australia Premier Steven Marshall said testing had identified five new cases in the past 24 hours, while 14 people were suspected to be infected and were awaiting test results.

"We are not out of the woods. We are just at the beginning stages of dealing with this particular very nasty cluster," Mr Marshall told reporters in Adelaide.

Mr Marshall said all cases could be traced back to an Australian who arrived in South Australia from overseas on Nov 2 and entered mandatory quarantine in a hotel.

Hotel workers are believed to have contacted the virus after touching a surface contaminated with the virus, Mr Marshall said.

Authorities fear the virus could have spread beyond hotel workers and their close contacts, prompting a mandate that confined about 4,000 people to their homes.

"This is a very, very worrying situation. I'm not going to underestimate the concern that I've got about this," said Dr Nicola Spurrier, South Australia's chief public health officer.

Australia has recorded about 27,800 infections and 906 deaths, far fewer than many other developed nations.

The bulk of the infections were recorded in Victoria state, which forced nearly 5 million people into a stringent lockdown for more than 100 days after a surge in cases.

The outbreak has now been contained, with Victoria recording no new cases for the last 16 days.