Ewen McKenzie remains winless in his first three games in the Wallaby hotseat, four penalties from the boot of Christian Leali'ifano all they had to show against a powerful South Africa side.
This was the eighth time that the two sides have met in Brisbane with Australia winning on all seven previous occasions but it was the visitors who dominated from the off.
Oosthuizen, on as a blood replacement early on, crashed over after a driving maul with only five minutes on the clock, Steyn adding the extras.
Christian Leali'fano responded with an eighth-minute penalty but the impressive Steyn continued his accurate kicking during this year's Rugby Championship.
The Springbok physicality was proving more than the Wallabies could handle at times and Steyn was more than happy to punish any subsequent indiscipline as the first half wore on.
Three penalties from his boot to one more from Leali'ifano meant the Springboks went in at the break 16-6 to the good, the South African missing a long-range effort with the final effort of the first period.
After the break the Wallabies needed to start quickly and when le Roux was collared behind his own try-line the home side had a five-metre penalty to build from.
The Springboks then conceded the penalty and Australia skipper for the day Will Genia opted for the points, Leali'ifano obliging to close to the gap to only seven points at 16-9.
But no sooner had McKenzie's side got a foothold than they were right up against it after a bizarre passage of play involving Bryan Habana and Michael Hooper.
Habana took a quick-tap penalty that referee George Clancy was not happy with but after the whistle had been blown Hooper upended him and found himself, possibly somewhat harshly, in the sin bin for a dangerous tackle.
Steyn slotted the subsequent penalty but despite their numerical disadvantage it was the Wallabies who scored next, Leali'ifano again unerring from the kicking tee to close the gap to 19-12.
One converted try separated the two sides so the next score was always going to be crucial and it was the so-far unbeaten South Africans who provided it.
Only a minute before Hooper could return to the field Habana's searing break and chip over the top down the left was recycled by the Springboks.
And before the Wallabies could re-organise captain de Villiers had busted through for the vital score, Steyn was uncharacteristically off target with the conversion but the visitors were 24-12 to the good.
That ended the game as a contest but the South Africans were far from finished.
Kirchner went over in the right hand corner on 65 minutes after a superb pass from Ruan Pienaar.
And three minutes later a le Roux dummy parted the Australian backline and he too was over for the bonus-point score.
There were still ten minutes to go as the Wallabies desperately tried to salvage some pride but the impressive Springbok defence held firm to secure their biggest ever win on Australian soil.