Acting Chief Medical Officer Professor Paul Kelly said the Prime Minister had accepted the AHPPC's advice on further measures at quarantine centres.

"From now, all states and territories have agreed that anyone with quarantine facilities, anyone working in those quarantine facilities will be tested at least weekly to make sure that we increase that surveillance in that area," Professor Kelly said.

"[Hotel quarantine] is our major risk now of reintroduction of COVID-19 into Australia, as we have seen and Adelaide over recent days."

Professor Kelly said Australians returning from countries with high case numbers, such as the UK, USA and India, posed the greatest risk.

"The hotel quarantine is our main game. When you think about what's happening overseas, over 55 million cases now, enormous numbers in many countries and that is where Australians are coming back from," he said.

"We expect that some of those people that come back at this time will be infected and indeed can be infectious, so anyone in close contact, even with the very best precautions, can be at risk."

Professor Kelly made note of the swift action of the South Australian government and commended the work of contact tracers in the state. He said he was in 'constant' communication with South Australia CHO Professor Nicola Spurrier, alongside daily meetings with the AHPPC.

"The news today really is that as the South Australian authorities have done, with their public health response, they have gone fast and gone hard, they have gone broad," he said.

"They are getting onto the cases they have found so far very quickly, large numbers of people turning up for testing, the testing is being done very quickly in laboratory."

He said he'd spoken with the Australian Defence Force on Wednesday to increase their capabilities in South Australia to assist with testing and contact tracing.

"[South Australia] does need to increase their ability to get tested and they are certainly getting onto that today and ADF resources will be thrown in to assist."

South Australian premier Steven Marshall announced a severe six-day lockdown aimed to be a 'circuit breaker' against the outbreak.

"There is no second chance to stop a second wave," Mr Marshall said on Wednesday.

When asked if South Australia's six-day pause was based on AHPPC advice Professor Kelly said states and territories alone were in charge of the decisions made to curb the virus.

"The AHPPC was not involved in that decision-making today, that was a matter for the South Australian government which they told us about today," he said.

"It is their decision as they are able to do within their own jurisdiction, they are the elected government."