Premier Steven Marshall says officials have been carefully monitoring the sharp rise in cases right across the country.

"What we are very concerned about is the very significant increase in transmissibility. We do have to take action," he said on Friday.

"We've updated our forecasts here in South Australia and if we don't take action there is going to be a significant problem for our country.

"The sheer volume of cases could overwhelm health systems right around the world."

Gene sequencing on the SA outbreak has revealed 72 per cent of the new cases are now the Omicron strain.

In response, SA has removed a ban on the widespread use of rapid antigen tests, but only as a screening tool for people without symptoms.

It has also welcomed the decision to cut the interval between a second dose and a booster shot to four months.

For at least the next four weeks people are encouraged to work from home.

A range of local restrictions that were due to be eased on December 28, including changes to density limits and caps on private gatherings, will stay in place into the new year with a date to be determined.

Mr Marshall said there was no way they could be relaxed in the current circumstances.

Over Christmas and the new year, South Australians have been urged to limit celebrations and ensure they keep to the rules, including the 30-person cap on family gatherings.

In relation to the rapid antigen tests, the government has recommended their use before entering high-risk settings such as nursing homes, before attending crowded venues or events, before going to high-risk worksites, or as reassurance for asymptomatic people feeling anxious in the current COVID climate.

Anyone who tests positive on a rapid antigen test must get a more conclusive PCR test and isolate until they return a negative result.

SA officials hope the new tests will reduce demand on the state's virus testing centres which have been swamped in recent days, blowing out waiting times to more than eight hours in some cases.

Chief Public Health Officer Nicola Spurrier said to help cut the lines further SA would remove the need for casual contacts of coronavirus cases to have tests unless they develop symptoms.

Instead of three tests, close contacts will only be required to have an initial test and then one at the end of their quarantine period.

"It's really important when we're looking at the whole response to this pandemic that we focus our testing where we get the most bang for our buck," she said.

Professor Spurrier said eight people were currently in hospital with one man in his 30s on a ventilator in intensive care.

SA now has 2917 active infections with 1886 of those isolating at home.

Australian Associated Press