Travellers to South Australia from Greater Brisbane will be required to quarantine for 14 days in response to that city's three-day lockdown over the latest coronavirus scare.
The restrictions will take effect from Saturday, while anyone who has arrived since January 2 will be required to be tested three times, but will not need to isolate.
The measures follow the shutdown ordered by the Queensland government after a cleaner at a quarantine hotel was diagnosed with the more contagious UK variant of COVID-19.
Premier Steven Marshall said SA would not hesitate to impose tighter restrictions on Queensland if necessary, while Chief Public Health Officer Nicola Spurrier said the latest developments were worrying.
She urged all South Australia to reconsider any travel plans to Queensland.
"We'll keep watching the situation. We are going to use that hotspot model, at least for the time being," Professor Spurrier said.
"But if anything changes, it may be that anybody returning from anywhere in Queensland has to quarantine here.
"And as things get worse it may be that we have to reduce the number of people coming into our state."
Prof Spurrier also confirmed that genomic testing had now revealed four returned travellers to SA had been found to have the UK strain of the virus.
Two of those have since been cleared of the disease and released from hotel quarantine, while two were still in isolation including one who may be an old infection.
That has prompted SA to introduce daily saliva testing from Friday for all workers in the hotel quarantine system on top of the weekly, and more accurate, throat and nasal swabs.
Prof Spurrier said everything was being done to improve the hotel quarantine system in SA and make it safe.
"The problem with having quarantine in a hotel is that it is inherently risky," she said.
"But we have done the best we possibly can. At the end of the day, it is not a hospital."
"Hand on heart, this is the safest we can possibly be."
SA health officials continue to monitor the situation with COVID-19 clusters in both Sydney and Melbourne, but no changes to border restrictions have been announced.
Travellers from NSW are required to quarantine while the border remains open to people from Victoria.
Police Assistant Commissioner Linda Williams said extra border checkpoints would be introduced in SA's north to watch for people who may have travelled from Brisbane.
South Australia reported three new coronavirus infections on Friday, all returned travellers in hotel quarantine.
