South Australia has gone "hard and early" with a range of renewed COVID-19 restrictions as officials battle to contain a rapidly spreading and worrying cluster of infections in the state's "biggest test to date".
Premier Steven Marshall says restrictions on pubs, restaurants and gyms and limits on various gatherings will come into effect from midnight and remain for at least two weeks.
He says coronavirus testing stations will operate on extended hours and the state's contact tracing resources have been increased.
"We are now facing our biggest test to date," Mr Marshall said.
"We are working around the clock to stay ahead of this cluster. No effort will be spared to slow and stop the spread.
"Time is now of the essence. We cannot wait to see how bad this gets.
"We don't want to keep these new and changed restrictions in place for one day longer than we need to, but the health advice is very clear. We need to act hard and early."
Chief Public Health Officer Nicola Spurrier said the number of cases linked to the Parafield cluster, in Adelaide's north, had not increased on Monday.
But three children from the family at the centre of the outbreak were showing symptoms despite initially testing negative.
The cluster is put at 17, with cases ranging from a one-year-old to two people in their 80s, and takes the total number of active infections in South Australia to 34.
The other 17 are returned, overseas travellers.
All are in hotel quarantine, except for two from the new outbreak who have been admitted to the Royal Adelaide Hospital as a precaution because of their age.
The cluster has been linked to one of Adelaide's quarantine hotels with three staff, including two security guards, among those testing positive.
All staff at the quarantine hotels will now be tested weekly.
Prof Spurrier said the guards were employed by a private company but police and SA Health staff were present at all times and officials were confident in the secure operation of the facilities.
She said it was difficult for her to compare SA's hotel quarantine measures with those which failed in Victoria.
Nevertheless, she said South Australia was facing the prospect of a second surge of COVID-19 cases.
"It looks like we're in the beginning or the early part of what could be called a second wave," Prof Spurrier said.
"But we have time to get on top of it."
The cluster closed down two schools in Adelaide's north along with a Hungry Jack's restaurant at Port Adelaide.
An Anglicare aged care centre at suburban Brompton was also placed in lockdown after two nurses, a mother and daughter, tested positive.
A pop-up testing station was established at the Parafield Airport with hundreds flocking to it and other testing centres, prompting delays of several hours.
The outbreak prompted Western Australia, Tasmania, Queensland and the Northern Territory to reimpose border restrictions requiring people arriving from SA to quarantine.
Victoria said it would screen and test people on arrival, while NSW and the ACT made no changes but were monitoring the situation.
The cluster emerged on Sunday after an 81-year-old woman went to Adelaide's Lyell McEwin Hospital for testing and was hospitalised.
Two members of her family also tested positive, a woman in her 50s and a man in his 60s, including one who worked in the quarantine hotel.
Warnings were issued for a range of public bus services and various locations including a swimming centre, fruit shop, two supermarkets, a chemist, a GP surgery, a department store and a hotel in the centre of Adelaide city.
People there at various times were being asked to monitor for symptoms.