Quinton de Kock's superb hundred put South Africa in firm command of the second Test against Australia.

The Proteas can wrap up an overseas series triumph in Hobart this week and De Kock's 104 from 143 balls, along with Temba Bavuma's stoic 74, gave the tourists a 241-run first-innings lead after they were all out for 326.

Kyle Abbott needed only four balls to make inroads into Australia's fragile batting line-up as Joe Burns was strangled down the legside, but Usman Khawaja's battling 56 not out lifted the hosts to 121 for two before bad light brought an early end to proceedings.

While Steve Smith's men made a better fist of their second innings after being skittled for an embarrassing 85 on Saturday, they face a mighty task over the next two days to avoid a fifth straight Test loss which would hand South Africa an unassailable 2-0 lead in this three-Test series.

Defeat would also signify Australia's first series loss at home under Darren Lehmann, putting heavy pressure on the coach and captain Smith.

Smith was there at the close on 18 after Khawaja and David Warner had steadied the ship with a 79-run stand following Burns' early dismissal.

Warner was typically counter-attacking before elbowing a delivery from Abbott onto his stumps five short of a fifty.

Khawaja did get to a half-century on the back of some sumptuous drives through the cover region but, with Australia staring at a 120-run deficit overnight, he still has plenty of work to do.

The size of South Africa's lead was down to the brilliance of De Kock, who, in partnership with the obdurate Bavuma, put Australia to the sword in the morning session, following Sunday's washout.

Khawaja was unbeaten on 56 when bad light stopped play (Getty)

Resuming on 171 for five, De Kock took the attack to the hosts' bowling and peppered the boundaries on both sides of the wickets.

He passed his fifty shortly after Bavuma, but soon overtook his partner and went to three figures in just 69 more balls, striking eight boundaries.

The left-hander eventually went when he missed a straight one from Josh Hazlewood, but by then the pair had added 144 for the sixth wicket and took the lead to 191.

There was to be no century for Bavuma, though, as he fended a short one from Hazlewood, but his 74 - his third highest Test score - was a crucial innings.

Hazlewood soon wrapped up the tail - not before some last-wicket hitting from Vernon Philander - to finish with figures of six for 89.