Melbourne's surge in coronavirus cases is a "clear threat" to South Australia with the state government imposing a hard border closure with Victoria to prevent its spread to SA.
From midnight on Wednesday, only South Australians coming home and essential travellers will be allowed to cross into SA from Victoria.
Extra police resources will be sent to patrol the border with some roads to be physically blocked.
Defence force support is being considered.
SA will also maintain its 14-day quarantine measures for people coming from NSW and the ACT but has set July 20 as the possible date when they can be lifted.
Health Minister Stephen Wade said it was now 105 days since SA's last case of COVID-19 community transmission without a known source.
"At the same time over the last two weeks we've watched with increasing concern as the outbreak in Victoria has grown with a ten-fold increase in new cases," Mr Wade said.
"Victoria is now a clear threat to public health in South Australia."
Police Commissioner Grants Stevens said like most jurisdictions, SA had been forced to review its attitudes to people coming from Victoria.
He said authorities were also mindful that many restrictions within SA had recently been lifted, allowing locals more freedom and the resumption of many activities.
"That level of movement within our own community does create risk if COVID-19 finds its way in," the commissioner said.
"The greatest risk of it finding its way in at the moment is through Victorian people travelling into South Australia."
The tougher restrictions are likely to remain in place for at least two weeks.
SA reported no new virus cases on Tuesday, leaving its total at 443 with only three of those considered active.
But Chief Public Health Officer Nicola Spurrier said it was unrealistic to think SA would not have new infections at some point as she called for more people to get tested.
"I can understand why people are relaxing, it doesn't look like there's a problem," she said.
"There is a problem, we can see it happening in Victoria and we could have something similar here.
"The only way I can get on top of it is if people are getting tested."
Professor Spurrier said the rise in Victorian cases was "absolutely worrying" and SA would need to see an end to community transmission there before lifting border restrictions.
Australian Associated Press