South Australia is going into a six-day lockdown as a "circuit-breaker" to help get on top of a COVID-19 outbreak in Adelaide.

South Australia will go into lockdown for six days, with a range of restrictions to provide a "circuit-breaker" and help control a worrying cluster of COVID-19 cases in Adelaide.

Imposing some of Australia's swiftest and strictest measures during the global pandemic, Premier Steven Marshall says pausing most community activity will significantly reduce the risk of the virus spreading further.

It could also prevent the need for a much longer Victorian-style shutdown.

"We need this circuit breaker, this community pause. We are at a critical point, but we will get through this," the premier said on Wednesday.

"We are really asking the people of South Australia to join with us on this mighty quest to stop this disease in its tracks.

"We've got one chance with this."

Police Commissioner Grant Stevens said the six-day period would be followed by another eight days of continuing restrictions.

From midnight all schools will close along with universities, pubs, cafes, food courts and takeaway food outlets.

Regional travel will be banned and aged care centres will go into lockdown.

Factories will close, along with the construction industry, and elective surgery will cease.

Wedding and funerals will be banned along with all outdoor sport and exercise and masks will be required outside the home.

People will only be allowed to leave their homes once each day to buy groceries or to seek a COVID-19 test or other medical treatment.

Supermarkets, petrol stations, medical centres, critical infrastructure, public transport, airport and freight services, banks, post offices, school and childcare for essential workers and veterinary services will be allowed.

The business sector has backed the lockdown but said it could be devastating for the state economy.

Chief Public Health Officer Nicola Spurrier said the so-called Parafield cluster had only grown by two cases to 22 overnight. Seven more people, thought likely to be infected, are waiting on test results.

However, she said there were worrying signs about the spread of the virus through the community, with information suggesting the particular strain had a short incubation period.

"Clearly, if it's reintroduced to the community it takes off very quickly," Prof Spurrier said.

"That's exactly what happened in Victoria. I don't want that to happen in South Australia and I'm going to do everything possible to make sure that doesn't happen.

"I would really like to not have any further cases here, but I think it's inevitable that we will.

"But I want all of those chains of transmission to stop."

With immediate signs of panic-buying across Adelaide's supermarkets, Mr Stevens urged people to stay calm and be assured that supplies of all necessary items would be maintained.

But he said police could only do so much to enforce the lockdown and its restrictions.

"Our expectation is people will do the right thing and will abide by this extreme level of imposition for a short period of time and help us do our job," he said.

"Clearly, if you are out and about during these six days you should have the ability to justify the reason for your travel.

"But essentially, you are required to stay at home for the next six days."

Thousands of people continued to flock to testing stations on Wednesday after 5000 tests were conducted on Monday and more than 6000 on Tuesday.

Efforts were being made to extend the hours of operation by bringing in more staff, including some from interstate.

A number of sites across Adelaide are of key concern, particularly a pizza bar, where a worker tested positive, two northern suburbs schools, a hospital and a swimming centre.

People who visited those sites have been asked to quarantine and get tested.

Almost 40 other locations are listed as places confirmed cases have visited in recent days, with people there at the same time asked to monitor for symptoms and get tested if they feel ill.