South Australia will close its border to greater Sydney after a coronavirus cluster in the NSW capital jumped to 70 cases.
Premier Steven Marshall says the border will shut from midnight on Sunday and checkpoints will be set up to test people for COVID-19 at NSW road border crossings and at Adelaide Airport.
All people who have been to greater Sydney will be ordered to undertake 14-days of self-quarantine on arrival in SA, while residents of Sydney's northern beaches residents will be turned away at the border.
"We know this is going to significantly affect Christmas travel plans, so we don't take these decisions lightly, but in this instance we believe that this is the best way that we can protect South Australia from any seeding into our state," Mr Marshall told reporters on Sunday.
Chief Health Officer Nicola Spurrier said people from regional NSW will still be able to enter SA without going into self-quarantine as long as they haven't been in Sydney after December 10.
She said any SA residents currently visiting or returning from the greater Sydney area must undergo self-quarantine at their homes over the Christmas period.
"Unfortunately it will mean that the family will not be able to get together at Christmas because we cannot take that risk," Dr Spurrier said.
"It is hopefully some relief though for people from South Australia who happen to be in NSW at the moment, so they will be able to come home, but obviously it's going to be very disruptive for many people with their Christmas plans."
The chief health officer said while there is increasing concern about the NSW outbreak, contact-tracers in that state are "absolutely gold-standard".
With 30 more virus cases reported on Sunday, the Sydney outbreak has grown to 70 confirmed infections.
SA recorded just three new cases, all of which were already in hotel quarantine, and is now less than a week away from declaring its recent Parafield cluster officially over.
Authorities consider two incubation cycles, or 28 days, the necessary time frame before an outbreak can be declared eliminated.
Dr Spurrier encouraged South Australians to get tested for COVID-19 and said virus testing stations were being expanded into rural and regional areas.
She said that the state's hotel quarantine system has also been tightened ahead of a new group of international arrivals later this week.
"There's always a risk when you have this nasty virus in your state, but I truly believe that we have gone above and beyond to make this as safe as possible for South Australia because I can tell you, I personally do not want to go through another Parafield cluster," she said.