Chief Public Health Officer Nicola Spurrier says a man in his 20s who went to Victoria recently and returned to SA on July 26 has tested positive for the virus.

His case follows that of a woman, also in her 20s, who tested positive on Tuesday after relocating from Melbourne to take up a new job.

Professor Spurrier says the two cases show the risk posed by people travelling to Victoria, where more than 700 new infections were reported on Thursday.

"There may be people that are seeking essential traveller status so they can go to Victoria and return," she said.

"But I'm urging all South Australians to think very, very carefully about any need to travel to Victoria at this time."

The man travelled back to Adelaide by road and is now in hotel quarantine.

It's not known what work he went to perform across the border.

But Prof Spurrier said officials had identified only one close contact who was now in isolation and did not believe the case posed a threat to the wider community.

"I am very confident this does not pose any risk at all for South Australians," she said.

"What it does show is a very real risk in Victoria, with a high case number today and that level of community transmission."

Thursday's new case takes the total number of infections in SA since the start of the pandemic to 449.

SA also conducted more than 4700 tests for COVID-19 on Wednesday and officials continued to urge as many people as possible to get checked.

On Friday, the transition committee will look again at the state's border restrictions and other measures but is unlikely to make any changes.

Under the current arrangements, only essential travellers can come into SA from Victoria and anyone coming from NSW and the ACT must quarantine for 14 days.

Premier Steven Marshall said a number of clusters in NSW remained concerning.

"They're keeping their numbers down below 20 but 20 can turn into 40 and 40 can turn into 80 very quickly," he said.

"So if need to take action to strengthen the border with NSW then that's precisely what we'll do."

Australian Associated Press