The state of South Australia announced a six-day "circuit-breaker" lockdown for its nearly two million people on Wednesday to contain a sudden coronavirus cluster in its capital city that ended a months-long streak of no infections. 

Schools, restaurants and factories were told to close at midnight while stay-at-home orders were issued for residents across the state.

It came as two new cases were linked to a cluster that emerged from an Adelaide hotel used to quarantine travellers from overseas, taking the outbreak to 22 cases. 

Weddings and funerals will be banned and mask-wearing in public made mandatory in the state, which had not recorded a significant outbreak since April.

"We are going hard and we are going early. Time is of the essence and we must act swiftly and decisively. We cannot wait to see how bad this becomes," state premier Steven Marshall said.

People have been told only to leave their homes for essential work, to buy groceries or for health reasons.

Chief health officer Nicola Spurrier said the "extreme" measures would give the state of 1.8 million people time to get on top of contact tracing and halt chains of transmission in their tracks.

"I cannot be making this decision in two or three weeks' time or even two or three days' time because it is going to be too late," she said.

On Monday, as 17 cases were confirmed, officials had begun ordering thousands of suspected close contacts to self-isolate and suspended international flights.

The new restrictions come amid fears the latest outbreak has the potential to infect high-risk populations, with care workers and a prison guard among those testing positive.

Spurrier said South Australia was hoping to avoid a lengthy Melbourne-style lockdown, where residents in Australia's second-largest city spent months mostly confined to their homes after security bungles at a hotel quarantine.

Melbourne recorded thousands of cases and hundreds of deaths but has begun easing restrictions after more than two weeks without any new cases.

Other states, where the virus has largely been stamped out, have imposed new quarantine rules on anyone travelling from South Australia.

The country's internal borders had been gradually reopening and were due to be almost fully reopened by Christmas.

Australia has been relatively successful in containing the virus, with just over 27,700 cases and 907 deaths recorded since the pandemic began.