Seventeen coronavirus cases have now been linked to a cluster in Adelaide's northern suburbs, South Australia's Chief Public Health Officer says.
They are all linked to four community cases announced on Sunday, and are outside hotel quarantine,
The new cases are South Australia's first without a known source of transmission since 16 April.
The first case to be identified was a woman in her 80s. She was found to have the virus after being treated at the Lyell McEwin Hospital in Adelaide's northern suburbs.
Since she tested positive, two of her close contacts a woman in her 50s and a man in his 60s, have also tested positive. One of them is the elderly woman's child.
A fourth case linked to the cluster was identified on Sunday, of a person who works at Yatala Labour Prison in Adelaide's northern suburbs and is a close family contact of one of the earlier cases.
Chief Public Health Officer Dr Nicola Spurrier has said that a medi-hotel where one of the infected people worked is the likely source.
Medi-hotels are hotels used by incoming travellers and local residents who can't safely quarantine at home.
"We haven't got the genomics yet, but I'm absolutely certain it has come from a medi-hotel," she said.
Two of the infected people are in the Royal Adelaide Hospital, which Dr Spurrier said was because of their age not their health, while the others are in hotels.
About 90 people who were in the Lyell McEwin Hospital emergency department at the same time as the elderly woman have already been told they must quarantine.
And the Department of Corrections said it was activating a health rapid response team to help with contact tracing efforts at the Yatala Labour Prison.
To help with this, all employees have been asked to stay at their posts.
An Adelaide primary school was closing on Monday to undergo a deep clean after a student there was identified as a close contact of one of the latest positive coronavirus cases. Parafield Plaza Supermarket has also closed.
All staff at South Australian quarantine hotels will now also be getting tested every week.
Officials aren't holding out hope this cluster will stop at four cases.
Dr Spurrier said on Sunday that more people were likely to end up getting infected.
She said this could be a wake-up call for South Australians: "If you have respiratory symptoms, you've got to get tested."
Western Australia has already changed its border rules because of the cluster.
Everyone who arrived at Perth Airport on Sunday from South Australia will need to get tested, if they haven't already. They also need to self-quarantine for 14 days.
Anyone who arrived in WA by road will need to follow the same self-quarantine and testing rules.
And anyone who arrived in the state by any method on Saturday or Sunday can expect to be contacted by WA officials. They'll need to be tested and self-quarantine until the results come back.
How other states and territories responded
Victoria has declared Adelaide a Covid-19 hotspot.
Premier Daniel Andrews said anyone arriving from South Australia at Melbourne airport will be interviewed, and may be required to undergo a rapid test.
He said state health authorities will have more details later in the day.
He also said Victoria will be ready to offer any assistance to SA.
Victoria today recorded its 17th straight day with no new coronavirus cases or deaths.
Anyone now in Tasmania who has been in South Australia since last Monday, 9 November, has been asked to self-isolate.
Anyone who arrives in Tasmania from SA from today will be required to quarantine.
Tasmania's Public Health Service also said those who have recently been in South Australia should be alert for Covid-19 symptoms.
A further announcement will be made later today.
The Northern Territory Chief Minister announced all arrivals from South Australia will have to undergo supervised quarantine.
The declaration of South Australia as a hotspot for the purposes of travel to the NT will take effect immediately.
People who arrive in the NT will have the choice to return to South Australia immediately instead of undergoing quarantine.
Anyone who arrives today or tomorrow will not have to pay the $2500 fee to undertake quarantine.
Two planes are due to arrive in Darwin from Adelaide today.
Anyone who has arrived at Perth Airport from South Australia from yesterday is being been tested for Covid-19, either on arrival or within 24 hours of arrival and told to self-quarantine for 14 days.
These arrivals will also be required to be tested on day 11 of their self-quarantine.
This includes passengers from a Qantas flight from Adelaide to Perth who flew in on Sunday.
People who entered the state by plane were given the option of flying back to Adelaide on Monday after being told about the changes on arrival.
Passengers due to arrive on a flight later on Sunday were told about the new requirements and given the option to disembark.
New South Wales
New South Wales is so far keeping its border open, with no required quarantine, for travellers coming from South Australia.
"Our health authorities are confident the South Australian Government is on top of it," NSW Premier Gladys Berejiklian said.
"You can't open and shut borders and change things overnight every time there's an outbreak.
"I'm not suggesting it's not concerning - it is."
Queensland Premier Annastacia Palaszczuk has tweeted that her government is assessing the South Australian Covid-19 outbreak and may impose restrictions.
An announcement is expected soon.