The case was revealed on Wednesday, more than two weeks after the outbreak first emerged and takes the number linked to the cluster to 22.

Chief Public Health Officer Nicola Spurrier said the positive test involved a man in his 20s who was the family member of a previously confirmed infection.

He was already in home quarantine with confirmation of his case coming after a relatively long incubation period.

Professor Spurrier said officials remained comfortable with the suggested incubation period of 14 days, but would continue to review the evidence, especially in relation to the Delta strain of the disease.

"This is a demonstration of how the virus can be quite different in different people. This person didn't have any symptoms," she said.

"But he had been completing his quarantine at home. There's no reason to suspect there's been any exposure."

The man has now been moved into Adelaide's dedicated facility for people with an infection, while three of his family members have been moved to a quarantine hotel and will be required to complete a further two weeks in isolation.

As well as the new local case, SA also picked up two more infections in returned travellers.

Three people remain in hospital, including a man in his 70s who has been admitted to intensive care but is not on a ventilator. His condition is listed as serious but stable.

Premier Steven Marshall said the new case in the local cluster was a concern, but the fact that they were in quarantine was also a "massive relief".

The cluster prompted a week-long lockdown across South Australia last month to help authorities bring it under control.

It was sparked by an 81-year-old man who had recently returned to Australia from Argentina and had quarantined in Sydney before travelling to Adelaide.

Genomic testing confirmed he contracted the disease in Sydney but the precise source remains unknown.

The new case also came as SA prepares to ease some local virus restrictions from Thursday morning, including relaxing the general density requirement in most venues to one person to every two square metres.

Sports competition can resume with spectator numbers at outdoor venues limited to 1000, but SA will keep mask-wearing in place for high-risk settings, high schools and most public places, including shopping centres.

Family gatherings will continue to be limited to 10 people, and weddings and funerals will stay at 50 people.

All food and drink consumption must be seated.

A crowd of 15,000 has also been approved for the AFL game at Adelaide Oval between Port Adelaide and the Adelaide Crows this weekend, something that has angered the state's hospitality sector.

A group known as the Hospo Owners Collective has called on the state government to outline a plan for a return to 75 per cent capacity in pubs, restaurants and other venues.

It also urged the government to "cease distracting the public from our plight with mentions of how much money has been distributed in grants".

"We all know it is a drop in the ocean stacked against the losses of last week let alone the costs we have borne in the last 12 months of restricted trade in the interest of public safety," it said in a Facebook post.

Australian Associated Press