South Australian Premier Steven Marshall has announced an immediate six-day lockdown in a bid to squash Adelaide's growing cluster.

Marshall said he needed to "go hard and go early" to stop coronavirus spreading further, announcing a six-day "circuit breaker" for the state.

"Time is of the essence and we must act swiftly and decisively. We cannot wait to see how bad this becomes," Marshall said.

"As of midnight tonight we need our community to pause for six days. A series of wide ranging restrictions will be implemented to significantly reduce mobility in the community to stop the spread, to stamp out this virus."

The Parafield cluster has grown rapidly after a cleaner at one of Adelaide's medi-hotels accidentally caught the virus from a surface before spreading it to her family.

Health authorities discovered the cluster on Friday night after the cleaner's elderly mother tested positive to coronavirus in hospital on Friday night.

"We must act swiftly and decisively on the health advice to stay ahead of the game. That health advice is that we need a circuit breaker.

"We need a circuit breaker to stay ahead of this. We need breathing space for a contact tracing let's to protect the elderly, to protect the vulnerable, to protect our entire community.

"That is why, today, I am asking you to rise to the challenge again."

Among the things that will be closed from midnight tonight are schools and universities, pubs, takeaway shops and cafes, factories and shops.

In addition, weddings and funerals will be banned for six days as is all outdoor sport and physical activity.

South Australia's chief public health officer Professor Nicola Spurrier has confirmed only two new cases for the state.

She described the two new cases as "small but critical".

Both cases are linked to the Parafield cluster, taking the number of cases linked to it to 22.

All cases have been able to be linked, Spurrier said.

"There are also an additional seven people who are either awaiting test results or we had an initial test that was negative but we are highly suspicious and treating them as infectious," she said.

Major shopping centres won't close across South Australia – but they will only be open for the "most basic" of supplies.

"As the police commissioner outlined, we need to stay at home as much as possible. So my expectation is that it will only be the most basic supplies that are going to be open and that is food, medical supplies and pharmaceuticals, so we will not be having our major shopping centres open," Spurrier said.

"Of course if there happens to be a supermarket there, we will need to get open."

Spurrier said the active coronavirus cases were presenting with very different infections than usual.

"This is still very early days in this cluster and I want to talk you through the rationale of why we are asking every South Australian to do this at this point in time," she said.

"All of the cases, as I said, the positive cases, have been linked, and that is a phenomenal effort and it means that we are very early at the beginning of this, and we have a very, short window of opportunity to close it down and stamp it out in our communities.

"This particular strain has had certain characteristics. It has a very, very short incubation period.

"That means when somebody gets exposed, it is taking 24 hours or even less for that person to become infectious to others and the other characteristic of the cases we have seen so far is they have had minimal symptoms and sometimes no symptoms but have been able to pass it other people.

"We also know, because of that characteristic, that what we call a generation is only about three days and a generation is when one case is passing it on to the next level, and then that level, so if they pass it onto two people, they will pass it onto other people, and that is your third generation."

Spurrier said health authorities had traced down to the fourth generation "but the fifth generation is out there in our community and at the moment we are contact tracing to get onto that fourth generation and that is the Woodville Pizza Bar."

Spurrier said there was "no time to wait" for the fifth and sixth generations to spread the virus around, hence the six day lockdown.

"If I thought about this all day and then told the police commissioner, the premier, tonight, we would already be 12 hours behind so we really need to act fast under this," she said.

Woodville Pizza Bar

Spurrier described why it was so important for any customers of Woodville Pizza Bar to isolate immediately, revealing the link between all of the state's cases.

"The one case that we received information about yesterday midday was of a young man who works at one of our medi-hotels, not the Peppers Waymouth where the other two cases have been," she said.

"It was at the Stamford and this person was not a security guard, was not a nurse or police officer but worked in the kitchen and that made us very concerned because we could not work out how on earth that person became infected.

"How were those two medi-hotels link? We made the link last night where we had a close contact of one of our security guards who was actively working part-time at the pizza bar and the case last night also worked in the pizza bar at the same time as the person who was at the Stamford went and got a pizza.

"We are absolutely certain … that is what happened.

"That cemented my fears that this virus is reading very, very rapidly. You have a short incubation period and you have three days as those generations move on.

"So I know this is an absolutely big ask but if we leave this any longer and if we have people moving around the community and having a lot of contact with other people, then we're going to be in this for the long haul and it would be like the experience in Victoria where we get increasing cases every single day and we have to go into a significant lockdown for a very long period of time to snuff it out and to get rid of every last bit of community tracing."

No more 'panic testing'

Spurrier thanked the thousands of South Australians who had turned up for testing this week, but called for an end to "panic testing".

The sheer number of people wanted to get tested for coronavirus has brought Adelaide's testing sites to their knees.

"We will prioritise our testing. We are going to prioritise it so that people who have been told to get tested … we want them to be the priority and we are working out a plan to make sure that happens," she said.

"The next priority is people who have been to any of the sites on the website.

"It tells you exactly what you need to do if you are in a category. Those with symptoms will need to get tested.

"Going down the list, we want anybody in our community who has symptoms to be tested but what we do not want at the moment is for people with no symptoms and no link at all to any of those venues… we do not want panic testing because that will just block up testing sites."

There were more than 4000 South Australians in isolation yesterday however Spurrier said there was "certainly more than that" today.

"I have not got an exact number. Of course, it is very nice to tally up numbers but really our focus has been getting hold of those people and getting them into quarantine," she said.

"We are really at the beginning of this in South Australia and I need everybody to basically find a safe place to be for the next six days and stay there as much as possible.

"Whether this is a Netflix blitz for some people, and I know it is going to be really difficult for many people, but we all need to be doing it and it is quite a different situation to earlier this year."