David Warner can continue in Test cricket for "as long as he wants", believes Australia team-mate Steve Smith after the batsman's double century against South Africa.

Amid speculation over Warner's future in the red-ball team, the New South Wales veteran became the eighth man to cross the 8,000-run boundary for his country in the game's oldest format.

After a slew of lacklustre performances, the 36-year-old's first Test century since facing New Zealand in 2020 reaffirmed his credentials, and Warner was only halted when he was forced to retire on 200 with cramp.

Smith, the seventh man to reach the 8,000-run mark, was full of plaudits for his partner at the crease, after they piled on the runs against the Proteas in Melbourne, and feels he is still in his prime.

"I don't see any reason why he can't continue playing," Smith said. "He saw the ball well today. He can play as long as he wants.

"He played exceptionally well from ball one yesterday. He looks like he has a lot of energy, his feet moved really nicely.

"Today, he just played beautifully. I think the more he started to cramp, the more shots he started playing. Everything came from the middle."

Australia finished day two at the MCG on 386 for three, with Warner and Cameron Green both retiring hurt. Smith was the last batsman dismissed, having shared in a partnership of 239 for the third wicket with Warner, before the retirements made it an unusual end to the day's play.

The batting efforts gave Australia an overnight lead of 197 runs, with Warner chiefly responsible.

"It was a great blow [to lose him] but nice to be on the other side for much of it," Smith said. "He's doing fine, he's fit. It was quite energy consuming.

"Obviously, it was a strange end to the day. It was hard to believe I was the last wicket. It was an interesting finish but a good day."