Steve Smith is unsure how long he will carry on playing for Australia after surpassing the great Don Bradman's tally of Test centuries at the SCG on Thursday.

Smith reached three figures for the 30th time in the longest format on day two of the third Test against South Africa, making 104 on his home ground before Australia closed on 475-4 with Usman Khawaja unbeaten on 195.

Only Ricky Ponting (41) and Steve Waugh (32) have more Test hundreds for Australia than the 33-year-old Smith, who moved level with Matthew Hayden's haul.

The former captain is not thinking about his future as he enjoys living in the moment.

When asked about his future, he said: "I don't know, we'll see. I'm enjoying it at the moment. I really can't say how long I'll play for, I'm not sure.

"Take it one tour at a time, enjoy it, enjoy training and trying to get better. Whilst I'm doing that I'm happy playing, but don't know how long it will last."

Australia will fancy their chances of securing a 3-0 whitewash of the Proteas in Sydney as they look certain to seal their place in the Test Championship final at The Oval in June.

Smith is relishing the opportunity to pass on his experience to his team-mates as he continues to rack up the runs.

"It's [about] trying to get better, help the team win games of cricket," he added. "I think we've played some really good cricket the last 12 months…we've got two really big hurdles in front of us with India and the Ashes.

"For me, it is trying to get better and trying to help some of the other batters coming through.

"I still try to help Marnus [Labuschagne] as much as possible, [Travis] Head, Cameron Green, Matty Renshaw, Marcus Harris…try to impart as much knowledge of conditions and ways to go about.

"If I can say something and you see that lightbulb go on and someone figures something out I get a big thrill out of that. I'll try and help them as much as I can."