South Australia has reported no new coronavirus cases on Thursday, from a bumper 12,000 tests conducted across the state.

The lack of additional cases comes as SA embarks on its strict six-day lockdown amid fears it was on the cusp of a deadly second wave of the virus with the emergence of the Parafield cluster in Adelaide’s northern suburbs.

Premier Steven Marshall said he was confident the state would have a normal Christmas, after it stamped out the worrying outbreak.

“We have woken up to a very different South Australia today,” he said on Thursday.

“What we do for the next six days will determine if we are able to successfully stare down the threat posed by this outbreak of COVID-19.”

South Australia’s chief public health officer Professor Nicola Spurrier said the state had 23 active cases, as well as a further 17 suspected infections.

There were 3200 people in quarantine, she said.

“What we are doing is putting a double ring fence around all of those people,” she said.

“If you imagine that all of those people have had close contacts, and then their contacts have had close contact.”

The Parafield cluster, which began after a cleaner picked it up while working in a medi-hotel in the Adelaide CBD and then transferred it to her family members, remained at 23 infections on Thursday.

From midnight Wednesday, schools and most businesses closed across South Australia. Residents must stay home, except for one person leaving their house once a day for essential shopping such as groceries.

Premier Steven Marshall announced the “circuit-breaker” lockdown at short notice on Wednesday. Photo: Getty

Mr Marshall said the pause in most community activity will significantly reduce the risk of the virus spreading further. It could also prevent the need for a much longer Victorian-style shutdown.

He said he was “very confident” the state would have a COVID-normal Christmas.

“This is the very best chance we’ve got. By working together over the six days,” he told the Nine Network earlier.

Universities, pubs, cafes, retail stores, food courts and takeaway food outlets have also closed across South Australia.

Regional travel is banned and aged care centres are in lockdown.

There is a halt on weddings and funerals are banned, while outdoor sport and exercise have been banned. Masks are required outside the home.

People who are not essential workers will only be allowed to leave their homes once each day to buy groceries or to seek a COVID-19 test or other medical treatment.

Supermarkets, petrol stations, medical centres, critical infrastructure, public transport, airport and freight services, banks, post offices, school and childcare for essential workers and veterinary services are open.

The business sector has backed the lockdown but said it could be devastating for the state economy.

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The shutdown also has the support of Prime Minister Scott Morrison, who described it as a “pre-emptive and temporary” strategy to keep South Australians safe.

“These are precautionary and temporary measures with a clear end date,” he said.

All being well, the lockdown will be replaced by eight further days of heavy but less stringent restrictions.

Mr Marshall said SA was battling a “nasty” strain of coronavirus, which is highly infectious, very likely to be transmitted from surfaces and has an incubation period of just 24 hours.

Elsewhere across Australia, Victoria posted its 20th consecutive day without new virus cases on Thursday. More than 17,000 tests were conducted in Victoria in the previous 24-hour period.

NSW had its 11th day on end without community transmission, although it did have five more cases in people in hotel quarantine. It conducted more than 20,000 tests in the relevant 24 hours.

-with AAP