Prime Minister Scott Morrison believes no hotel quarantine system can be made 100% fail-safe after COVID-19 escaped South Australia’s isolation regime.

The Adelaide outbreak has triggered a severe six-day lockdown, with 23 coronavirus cases now linked to the cluster.

There were no new infections recorded in SA on Thursday, but 17 people are suspected to have the disease.

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