South Africa captain Faf du Plessis believes his side can profit from Australia's off-field difficulties in the fourth and final Test after taking a 2-1 series lead.

The Proteas thumped the tourists by 322 runs in Cape Town, their opponents sheepish on Sunday's fourth day after Cameron Bancroft admitted a charge of ball tampering 24 hours earlier.

It has been an ill-tempered series with prior incidents involving David Warner, Quinton de Kock and Kagiso Rabada before Bancroft and Steve Smith became embroiled in the latest scandal, the Australia captain now banned by the ICC for the next match at the Wanderers having confirmed his part in the decision to manipulate the ball.

And, as Cricket Australia carries out its own investigation amid pressure from the public back home, Du Plessis aims to take full advantage.

"I believe we can put some more pressure on the Australian team with all the stuff that's going on off the field," he said of the fourth Test in his post-match presentation. "There's a lot for us to work with."

Despite the controversy that engulfed this match and had the crowd involved, Du Plessis insisted his players had not been affected.

"We are extremely motivated in this series," he added. "Since that defeat in the first Test match in Durban, our motivation levels have just doubled.

"The guys are extremely focused on putting in big performances. Although that was happening, it was pretty easy for us just to make sure we could tighten that screw on the Australian team."

Morne Morkel collected the man-of-the-match honours after his career-best 9-110 across the two innings, with the retiring bowler delighted to take 5-23 as Australia capitulated from 57-0 to 107 all out on Sunday.

"I was hoping that I could deliver something special. I'm glad it all came together today at this amazing venue with the support of these people," said Morkel, who reflected on another proud achievement having claimed his 300th wicket earlier in the Test.

"It was very special," he said. "As a young boy, you dream of those sort of things. To join that club makes me so happy."