Prime Minister Scott Morrison believes no hotel quarantine system can be made 100 per cent fail safe after the disease escaped South Australia’s isolation regime.

The Adelaide outbreak has triggered a severe six-day lockdown, with 23 cases linked to the cluster by late on Wednesday.

Mr Morrison said Australia had welcomed more than 400,000 returned citizens and permanent residents during the pandemic.

“We’ve had a lot of people coming through and we’ve had outbreaks in cases that have emerged from quarantine in many states during that time,” he told the Seven Network on Thursday.

“You can’t have a fail-safe 100 per cent system in every case.”

He said South Australia was putting in increased levels of defence based on former health secretary Jane Halton’s review of the hotel regime.

Adelaide’s outbreak has been traced back to a cleaner who caught the virus working at the Peppers on Waymouth. Photo: Getty

Melbourne’s deadly outbreak also spread after the virus escaped from the hotel quarantine system.

Mr Morrison and other senior Liberals were vocal critics of Victoria’s lengthy lockdown, ordered by Labor Premier Daniel Andrews. But on Thursday the PM backed the “unconventional” move by South Australian Premier Steven Marshall, a Liberal.

“I know this is unconventional to take that step so early, but at the same time, given the uncertainties, I can understand why he took that decision,” he said.

After it emerged that Adelaide’s outbreak also came from a breach in hotel quarantine, acting chief medical officer Paul Kelly said on Wednesday that all of the country’s hotel quarantine workers would be tested weekly for the virus.

Professor Kelly said hotel quarantine was Australia’s major risk of COVID-19 being reintroduced.

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There are more than 35,000 Australians stranded overseas trying to return. Under current rules, all must complete 14 days in quarantine in an approved hotel on arrival back in Australia.

The nation’s top medical experts will look at the risks associated with hotel quarantine after issues were raised with workers doing multiple jobs.

Cabinet minister Simon Birmingham said the scale of risk needed to be assessed.

“We can’t always eliminate every risk, but we ought to have a look at them, assess them, act on the health advice where it is clear,” he told the ABC.

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Under SA’s tough lockdown rules, schools and almost all industries closed from midnight on Wednesday. Weddings, funerals and outside exercise are temporarily banned.

Takeaway food outlets have been shut, along with pubs, cafes and food courts. Supermarkets and a handful of essential services remain open.

Mr Morrison said hoped the harsh measures would be short-lived and avoid a long lockdown similar to Melbourne.

“This is a precautionary, circuit-breaking action that they’ve taken in South Australia,” he said.

“There are some elements of what is occurring there where there are some real uncertainties, particularly about the potential of a different strand of the virus.”

-with AAP