Australia remains firmly in the hunt for victory in the third Test against South Africa, needing just four wickets to wrap up the tourists' first innings thanks to some exceptional bowling on a cloudy and rain-disrupted fourth day at the SCG.

Pat Cummins (3-29 from 14 overs) took two wickets in a venomous spell of bowling late on day four to leave the Proteas 6-149, still 326 runs behind Australia's first-innings total.

With Australia eyeing a perfect summer of five wins from five Tests, the hosts will need 14 wickets on the final day of a rain-disrupted Test to condemn South Africa to a third defeat on tour.

Captain Cummins may have arrived at the Sydney Cricket Ground on Saturday hoping to give Usman Khawaja a chance to knock off five runs needed for his first Test double century, but once again Sydney's weather intervened.

The day started almost five hours later than scheduled and just shy of 48 hours after the last ball was bowled in the Test, after rain wiped out the first session-and-a-half of day four following an early finish on day two and total washout of day three.

Cummins declared Australia's first innings on 4-475, stranding Khawaja on 195 and giving the bowling attack a day-and-a-half to bowl South Africa out twice.

Australia made up for lost time once play started, with plenty of action in the opening overs, including a spectacular slips catch from Steve Smith that was denied by the slimmest margin in a review by the third umpire.

It was Elgar who edged Josh Hazlewood to Smith low down, but despite the dive and the ball appearing to lodge into the hand of a diving Smith, Richard Kettleborough spotted the ball grazing the grass as he completed the catch to deny the Aussies.

It did not take long for the Australians to strike though, and Elgar was again prised out with a bouncer angled into him that he gloved to Carey for 15, giving Hazlewood his first wicket of the series and first since the opening Test of the summer against the West Indies.

The Proteas skipper has scored just 46 runs at an average of 9.2 this series, his frequent early failures plunging his team into trouble.

Fellow opener Sarel Erwee, whose average this series is not much better at 15, fell soon after, opting not to play at a Nathan Lyon ball that went on with the arm to clip the top of off stump and have him bowled for 18.

The very next over, Heinrich Klaasen — South Africa's third number three of the series — went the same way as his skipper, fending a short ball to Carey, this time off the bowling of Cummins, for 2, leaving the Proteas 3-37.

Injuries to pace bowling options Mitchell Starc and Cameron Green, plus the dropping of Scott Boland, left Cummins very light on seam options through the day, an issue highlighted by the fact such little play had been able to take place due to rain leaving the pitch less rough than expected.

That meant Ashton Agar, playing his first Test since 2017, bowled his first Test overs on Australian soil as second change, working in tandem with Lyon before tea.

Temba Bavuma has faced more deliveries than any South African batter in the series, and dug in for another solid start, making it to the tea break but ultimately falling soon after the resumption when he feathered a Hazlewood length ball through to Carey for 35.

And that delivery was the first sign of reverse swing for the Australians, bringing their two-pronged pace battery back into the attack.

Cummins then dismissed Khaya Zondo LBW for 39 with a beauty of a yorker, and was within a couple of millimetres of claiming a second wicket in the same over when he trapped Marco Jansen in front, only for the ball to pitch marginally outside leg.

Jansen endured a torrid spell in the middle, wearing a couple of balls to the body as Cummins alternated banging the ball in short with teasing yorkers to both him and Kyle Verreynne as part of a sublime spell of bowling.

It was an edge that accounted for Verreynne, who fended a ball that just left him off the pitch to Smith at first slip for 19 in the fifth over of the spell.