Matt Renshaw has tested positive for Covid shortly after being recalled to Australia's Test side but for now will continue to take part at the SCG.
Renshaw stood separately from team-mates during the anthems after reporting feeling unwell and was then seen sat away from the dugout on the boundary edge.
"Australian batter Matthew Renshaw reported feeling unwell shortly before play in the third Test match against South Africa in Sydney and was separated from the squad. Renshaw has since returned a positive RAT test. He will continue to take part in the match," a CA spokesperson said.
Peter Handscomb was earlier listed as an emergency fielder on Australia's team sheet, meaning he could be drafted in as Covid substitute if needed although he had already been added the previous day in case other non-playing squad members, including reserve batter Marcus Harris, are released for BBL duty. Handscomb, who did not play for Melbourne Renegades on Tuesday, has yet to arrive in Sydney.
Players are now permitted to continue in a match even if they test positive for Covid. Tahlia McGrath took part in the Commonwealth Games final against India last year in such a scenario while Matthew Wade was set to play against England at the T20 World Cup before that game was washed out.
On the eve of the Test, Pat Cummins had spoken about how it had been a much more normal summer after two years of various restrictions. "I still don't know how we haven't had Covid in our team yet now I think about," he said.
Renshaw is playing his first Test since 2018 having been listed to bat at No. 6 in Sydney with Australia opting against five specialist bowlers for the final Test.
Instead they have fielded just two frontline quicks, Cummins and Josh Hazlewood, alongside two spinners in the recalled Ashton Agar and Nathan Lyon.
During the 2018-19 series against Pakistan in the UAE, Renshaw had been in line for a place in the team before suffering a concussion in the warm-up match.