Australia captain Steve Smith is unsure if paceman Mitchell Starc will be fit to return for the final Ashes Test in Sydney.

Smith scored his 23rd Test century during the fourth Ashes clash on Saturday, frustrating England as Australia salvaged a draw at the MCG.

The hosts, who hold an unassailable 3-0 series lead, were dominated for most of the Test, with Starc's replacement – Jackson Bird – taking 0-108 in the first innings.

Speaking at the end of day five, Smith said it was too early to tell if Starc would be available to come back from his heel injury in Sydney.

"Not sure yet," he told Channel 9.

"We've still got a little while before that game, more time for him to recover. We'll see how he goes."

Smith (102 not out off 275 balls), David Warner (86 off 227) and Mitchell Marsh (unbeaten 29 off 166) all batted defensively for Australia.

The Australia captain said he decided late on day four that the hosts would have to play for a draw.

"[We decided] probably late yesterday afternoon when it started raining," Smith said.

"It was a pretty slow wicket and pretty tough to get guys out throughout the whole game.

"Reverse swing came into play, but it was still so slow, you couldn't really get both sides of the bat for that long so it was a nice wicket to bat on to not get out, but it was hard to score on at the same time."