South Australia's borders will remain open but all travellers from NSW, Victoria and the ACT will be required to have a test on arrival and isolate until a result is received.
They will need to have a further test on day six if they are still in the state.
They will not be able to visit high-risk settings such as aged care homes, disability care facilities and prisons.
"We remain extraordinarily concerned about the Omicron threat," SA Premier Steven Marshall said.
"This has nothing to do with the Delta variant."
There have been about 40 cases in SA since the borders reopened on November 23, including the state's first mystery infection reported yesterday.
"That is in line with the forecast — the model that was presented — so this has nothing to do with Delta, it has everything to do with the Omicron variant," the Premier said.
Five new cases were reported on Saturday, including three locally acquired cases and two cases acquired interstate.
All of them have been linked to previous cases.
International arrivals into South Australia have had to quarantine for 14 days since concerns were first raised about the Omicron variant last weekend.
Chief Public Health Officer Nicola Spurrier said seven people who flew in on an international flight into Sydney may have been exposed to Omicron and then continued on to South Australia.
All but one of them had been contacted, she said.
Interstate arrivals will still have to have a COVID-19 test 72 hours before entering South Australia.
Disagreement at committee meeting
Professor Spurrier would not say what advice she gave to the state's directions committee, which decides border restrictions.
However, Police Commissioner Grant Stevens said Professor Spurrier came to the meeting with a recommendation to close the state's borders but "there are a range of factors that have to be taken into account" and a consensus decision was made.
He said "significant spread" of Omicron interstate would be the next trigger on tightening border restrictions.
The committee met on Friday and Saturday, and Mr Marshall said there would be "extraordinarily frequent directions meetings" over the next week or two.
He later tweeted that 90 per cent of South Australians aged 12 and over had received at least one COVID-19 vaccination dose.
Queensland imposes quarantine requirements
Queensland declared Greater Adelaide a COVID-19 hotspot this morning.
From Sunday morning, anyone arriving in Queensland from Adelaide will have to quarantine for 14 days.
"People are going into those communities in South Australia with the virus and consequently infecting other people who have every right to travel to South Australia and to come back here and are eligible to do so, but it has meant that the virus has come here earlier than we hoped," Queensland Health Minister Yvette D'ath said.
Country residents only stopping at Adelaide Airport will be able to enter Queensland without quarantining.
Western Australia limited travel from South Australia to returning residents on Thursday, while the Northern Territory increased testing requirements.
More exposure sites announced
SA Health has added two close contact exposure sites — the Halfway Hotel in Beverley and the Hotel Crown in Victor Harbor.
On Friday night, it added three close contact exposure locations — cafes in Hyde Park and Victor Harbor and parts of the plane on a Qantas flight from Melbourne to Adelaide on Monday.
Lower risk exposure sites were also added in Fullarton, Semaphore, Wayville, Adelaide Airport, Hyde Park, Tea Tree Gully and the rest of the same Qantas flight.
A COVID-19 case visited the Victor Harbor cafe — Nino's — last Sunday but the owners only found out it was an exposure site this morning after seeing it on social media.
Professor Spurrier said it could take some time to assess the risk posed at certain sites.
"To get the information that is more fulsome so that we're giving people the better advice about the quarantine time and who needs to quarantine, it does take that little bit longer to pull all of that information together," she said.
You can find information on testing site hours and the nearest site to you on the SA Health website.
