South Australia has left its border open to travellers from NSW but warns it won't hesitate to act if Sydney's COVID-19 cluster continues to expand.
However people from the city's northern beaches and two other suburbs will face immediate restrictions with some being forced into hotel quarantine and others into home isolation.
Police Commissioner Grant Stevens says Sydney's northern peninsula has been declared a high community transmission zone.
He says people who recently travelled to SA and had visited Avalon RSL and Avalon Bowlo will be placed into hotel quarantine.
It's unclear at this stage how many, if any, that will involve.
Anyone else who visited those two locations who is still planning to travel to SA will not be permitted to enter.
Anyone travelling from the wider northern beaches area will be required to self-isolate for 14 days and get tested.
The same applies to people who have been in Sydney's Penrith or Lavender Bay since December 11.
Mr Stevens said he had a strong sense NSW officials were moving "aggressively" to contain the cluster.
"But this is pandemic, it is unpredictable and we have an obligation to make hard decisions and we will do that if we need to," he said.
"If this cluster grows beyond the northern beaches and is seen to have spread far more widely through the NSW community, that is certainly something we will have to take into account."
South Australia has also reinstated a requirement for all people coming from NSW to complete an online border travel application.
Checks are being made on people who may have come from Sydney to watch the first cricket Test between Australia and India, which started at Adelaide Oval on Thursday.
Everyone attending the Test over the next four days will be encouraged to wear masks, especially while away from their seats.
Chief Public Health Officer Nicola Spurrier said considerable work was being done to bring the Sydney cluster under control.
"It's very early in terms of this cluster and it's very localised," she said.
"If we see things popping up in other parts of NSW, we'll take that new information on board.
"But at the moment I'm not recommending any changes to our own restrictions here."
Premier Steven Marshall said South Australia would not entertain "kneejerk reactions" but wouldn't hesitate to take further steps if required.
"We will continue to listen to the health advice about what we need to do to keep our state safe," he said.
"But at the moment we are satisfied with the excellent work NSW Health are doing to essentially put a net over that cluster."
With 10 more cases reported on Friday, the Avalon outbreak has grown to 28 confirmed infections.
No new virus cases were reported in SA on Friday and the state has no active infections.
SA 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 timeframe before an outbreak is declared eliminated.