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 WA, Police Commissioner Grant Stevens says.
SA shut its border from late on Sunday night after a hotel quarantine worker in Perth tested positive for COVID-19.
It followed the decision by the Western Australian government to place Perth, the Peel region and the South West in a five-day lockdown.
Mr 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."
Premier Steven Marshall said acting "hard and fast" had been effective in controlling the spread of the virus in Australia.
"We have confidence that WA will get on top of this situation and that these restrictions won't be in place for too long," he said.
The new restrictions also 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.
That includes South Australians returning home from WA.
The approach taken by SA is more severe than other states, which also brought in travel restrictions.
Victoria, Northern Territory, Tasmania and Queensland closed its border to areas affected by the WA lockdown.
The closures are in response to a security guard at the Sheraton Four Points hotel in Perth testing positive.
Authorities believe the man has probably contracted the highly contagious UK variant of the virus.
It comes 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 one new COVID-19 case on Sunday, a man in his 40s who recently returned from overseas.
He is in hotel quarantine and is the state's only active infection.
