Michael Clarke insists Australia's Cape Town disaster is firmly behind them as they prepare for the start of the second Test against Proteas in Johannesburg on Thursday.

The tourists looked in a strong position to clinch victory in the first Test after bowling the Proteas out for 96 only to collapse spectacularly themselves to 47 all out in their second innings and eventually lose by eight wickets.

With only two matches in the series, Australia must secure victory at the Wanderers if they are to avoid defeat in a Test series in South Africa for the first time since 1970.

Clarke told a press conference: "We had a really long chat after the game and we made a pact with each other that once we left the ground we were going to look to this second Test match, and I think we've done that.

"All the guys have trained well over the last couple of days. We've got to have a red-hot crack to try to win this game and level the series, and that's what we'll be doing out there."

Another blow for Australia was the loss of Shaun Marsh to a back injury, opening the door for Usman Khawaja to win a fourth cap, 10 months after making his debut in the fifth Ashes Test against England.

Clarke confirmed the 24-year-old, the first Muslim to represent Australia at Test level, will be a straight replacement for Marsh at number three.

"I had a good think last night, what I think is best for the team in terms of helping us win the game, and right now the straight swap is probably the best and right thing," said the skipper.

"I'm confident he'll be able to slip straight in and have some success for us."

Khawaja has so far failed to really take his chance in the longest form of the game, managing only 118 runs in his three appearances, but he is confident there is better to come.

The 24-year-old said: "I've felt really comfortable in all three Test matches. I haven't put up a big score yet but I've really enjoyed my time out there. I've felt like everyone was really supportive and I've felt a part of the team, which is probably the biggest thing."

Clarke also hinted at other changes, with 18-year-old fast bowler Pat Cummins catching his captain's eye in the nets and possibly in line for a Test debut.

"He's done absolutely everything right," said Clarke.

"He's keen as mustard for an opportunity. He's swinging the ball away from the right-handers and he's bowling good pace. There's certainly a chance that he could be playing tomorrow."

Key to South Africa's success in Cape Town was the performance of their pace bowlers, with debutant Vernon Philander stealing the show with a superb haul of eight for 78.

Bowling coach Allan Donald is delighted with the options available to the Proteas, but he knows there is much work to be done if they are to achieve a series victory.

"Competition is healthy," Donald told www.cricket.co.za.

"I will put my hand up and say that I was a little concerned; I'm not concerned any more. What I have seen and what's out there is really exciting and the competition for places can only intensify from here.

"They're a good side (Australia), they are very good on paper and we expect them to bounce back heavily.

"We're going to focus on what we have to do and do well and try and win this Test series here in South Africa."