The state of South Australia is on high alert after 17 cases of coronavirus were identified - its first community transmission cases since April.
It is believed the cases are linked to a so-called Medihotel in Adelaide - hotels used by people who cannot safely quarantine at home.
A hotel quarantine breach led to Australia's second wave of the virus - focused around Melbourne, Victoria.
Australia has seen cases drop to near zero after tough lockdown measures.
On Saturday, the state of Western Australia opened its borders to South Australia, allowing free movement after seven months. But now Western Australia has said anyone who has arrived from South Australia must take a coronavirus test and isolate for 14 days regardless of the result.
With the exception of the Melbourne outbreak, which killed about 800 people, Australia is widely seen to have handled the virus successfully; though it has done so with strict travel restrictions, including the continued closure of international borders and severe restrictions on movement within the country, which some argue is unsustainable.