Western Australia will block anyone who has recently been in South Australia from entering the state unless they can secure an exemption, with Premier Mark McGowan warning he won't hesitate to reimpose hard borders.
Just days after reopening his state, Mr McGowan has closed WA's border to SA after a cluster of 17 locally-transmitted COVID-19 cases was detected in Adelaide.
Anyone who has been in SA in the past 14 days will now be prevented from flying into WA unless they have an exemption, under harsh new restrictions coming into effect from 6pm on Monday.
The exemption criteria include government officials, military personnel, transport, freight or logistics workers and people travelling on compassionate grounds.
Passengers on two flights due to arrive from SA later on Monday will be notified and screened at Adelaide Airport prior to boarding.
Similar restrictions will apply to road arrivals - including those travelling through SA from other states - from Wednesday evening.
"Bringing back the hard border altogether is under serious consideration," Mr McGowan told reporters, urging anyone from SA to stay away if possible.
"Especially given the number of people from South Australia that have been travelling around the country in the last week.
"We need to act in the interests of all West Australians. I won't take any unnecessary risks."
It comes as WA recorded one new case, a returned traveller from Indonesia in hotel quarantine. The state has 13 active cases.
Anyone arriving from SA at Perth Airport is already being tested on arrival and on day 11, and being directed to self-quarantine for 14 days at a suitable premise.
All of those tests have so far come back negative.
People driving across the Nullarbor have been ordered to self-quarantine and get tested on day two and day 11 of the 14-day period.
WA authorities are also contacting anyone who arrived from SA over the weekend to instruct them to get a COVID test and self-quarantine.
The cluster in SA is believed to have originated with a hotel quarantine worker infecting members of their family.
WA's hotel quarantine staff are currently subject to voluntary weekly COVID-19 testing but the government has indicated that may become compulsory.
The premier said he was conscious of the fact there was already 2400 people in hotel quarantine.
"We don't want to overload the system," he said.
"What's happened with Adelaide is there's been a mistake in hotel quarantine, and we've always said this is the risk - you have too many people in hotel quarantine, you have too many risks in a concentrated environment where you have security guards and cleaners and everyone else."
The outbreak in SA came less than 48 hours after WA's borders were finally reopened to all states and territories.
People from NSW and Victoria remain subject to a 14-day isolation period but other interstate arrivals are not required to quarantine.
Police reported few delays on Saturday as 2287 people crossed into WA by air and road.
Another 10 domestic flights with 1000 passengers were expected to arrive in the state on Sunday.