Possible exposure to the more virulent UK strain of coronavirus and the lack of detailed information led South Australia to act fast to close the border with Western Australia, forcing up to 2700 people into isolation.
SA shut its border from late on Sunday night after a hotel quarantine worker in Perth tested positive for COVID-19 with fears he has the UK variant.
It followed the decision by the WA government to place Perth, the Peel region and the South West in a five-day lockdown.
Police Commissioner Grant Stevens said as more information became available about the situation in Perth, it might be possible for SA to refine its border restrictions.
"We understand the imposition this has on people in terms of a lack of advance notice," he said.
"But we are dealing with a pandemic. We can't always provide a lag-time for people to sort their affairs out. As much as we try to provide consistent and timely advice, we are dealing with changing circumstances."
Premier Steven Marshall said while no-one wanted to impose border restrictions, acting "hard and fast" had been effective in controlling the spread of the virus in Australia.
"We're treating this situation extraordinarily carefully. We want to make sure we don't bring this disease into South Australia," he told reporters on Monday.
"We don't want to take chances. We want to do everything we can to keep the quality of life we have here in South Australia."
The new restrictions mean anyone who has entered SA from WA since January 26 must self-quarantine and get tested on days one, five and 12.
That's expected to impact on about 2700 arrivals since that time, and Mr Stevens said they were all being contacted by SA Health.
People can apply for permission to travel from WA to SA for essential purposes, including South Australians returning home.
The new measures are in response to a security guard at the Sheraton Four Points hotel in Perth testing positive.
They come only a day after SA lifted restrictions for travellers from the wider Sydney region.
That briefly left SA with no major travel restrictions with any state or territory.
SA reported no new cases on Monday while a case reported in a returned traveller on Sunday has now been reclassified as an old infection.
The state has no active infections.
Australian Associated Press