Khawaja makes it anniversary to remember

Usman Khawaja celebrated the one-year anniversary of his return to the Test team with his fifth century in 12 months, helping Australia to 4-475 after a wet day two at the SCG against South Africa.

Khawaja was on the verge of his maiden double century when rain stopped play on Thursday, unbeaten on 195 after he and Steve Smith both struck tons in the third Test.

Travis Head then hit 70 off 59 entertaining balls, continuing his swashbuckling summer in which he is striking at more than 95 with an average of 87.5.

With the series already secure, Australia are now eying a 3-0 whitewash and the chance to lock in a spot in June's World Test Championship final.

The hosts will aim to bat into the third day, potentially opening the door for a rare follow-on scenario if they can roll South Africa cheaply with more wet weather forecast.

In superb form since he was recalled last January, Khawaja has averaged more than 75 since his comeback.

Unable to hold down a spot for most of his Test career, he is enjoying one of the great late-career resurgences at age 36.

"He is at the top of his game," Smith said.

"He is scoring runs at will, batting beautifully.

"He, like Davey (Warner), can play for as long as they like. They are both just playing really well."

After resuming on 51 not out on Thursday, the left-hander worked the ball on both sides of the wicket and hit out-of-sorts spinner Keshav Maharaj over cover for six.

He helped take 14 off one Kagiso Rabada over in the middle session, cutting the South African quick for four and then twice guiding him to the third-man boundary.

The one chance he offered up also went for four, middling Rabada to Anrich Nortje at point before the ball brushed the quick's pants and flew to the rope.

"It was a great innings," Smith said.

"He played exceptionally well from ball one. He hit his areas, was nice and patient when he needed to be, played the spin well.

"Hopefully he can get 200 or even 300 (more runs) tomorrow."

Smith also looked at home against the Proteas' bowling, before becoming Maharaj's first victim of the series on 104.

He had earlier dispatched the finger-spinner over the legside rope and taken fellow tweaker Simon Harmer back down the ground, also pulling the quicks when they went short.

The former captain brought up his 30th Test century in that fashion, pulling Nortje for four and becoming the fourth Australian in history to reach the milestone.

Such was Australia's dominance that when Smith was caught and bowled by Maharaj (1-08), it was hard to tell if the batter, bowler or SCG crowd were more shocked.

"It was a sigh of relief more than anything else," Maharaj admitted.

The Proteas' relief was short-lived when Head walked out.

He hit eight boundaries all around the ground and a big six back over Maharaj's head, before eventually being caught pulling Rabada (1-119) towards the square-leg boundary.

South Africa's long day in the field adds to their tour of woe, with Maharaj and Rabada both poor, and Nortje the only threatening bowler with two wickets on day one.