Du Plessis had ball-tampering suspicions prior to Cape Town

South Africa captain Faf du Plessis believes Australia may have been guilty of ball tampering before the revelations at Newlands.

Faf du Plessis says the amount of reverse swing produced by Australia aroused his suspicions of potential ball tampering before the dramatic events in Cape Town.

Steve Smith, David Warner and Cameron Bancroft were all suspended by Cricket Australia after the trio were found guilty of concocting a plot to illegally change the condition of the ball on day three of the Newlands Test last week.

The scandal has rocked the cricketing world and has overshadowed the four-match series, which South Africa lead 2-1 ahead of the final match at the Wanderers, beginning on Friday.

Captain Smith and vice-captain Warner were banned for a year while Bancroft received a nine-month suspension, and it was announced on Thursday that head coach Darren Lehmann will resign after the fourth Test.

Upon returning to Sydney on Thursday, an emotional Smith was asked whether he was aware of similar antics in other matches during his captaincy, to which he replied: "To my knowledge this has never happened before. This is the first time I've seen this happen and I can assure you it'll never be happening again."

Facing the media ahead of the Johannesburg meeting, Du Plessis confirmed he had previously thought that Australia may be breaking the laws in order to gain an unfair advantage.

"I thought so," he said. "The ball has been reversing quite a bit...I joked about it at the end of the last Test match that I've never seen so many guys put their hand up to open the batting!

"Usually it's green wickets and spicy in the beginning but now the big challenge in this series is coming in when the ball's tailing around, whether that's after 30 overs or 50 overs, but that's the nature [of it].

"Without having any evidence of it, we thought there's no way that the ball can go so early. It's just unheard of for a South African series where the ball is [reversing] this much.

"We try and do the same, we try and get that ball to talk as much as possible. But we certainly don't walk around with sandpaper in our pockets."

Smith, Bancroft and Warner have all returned to Australia and apologised for their actions - the former duo facing reporters in official news conferences while Warner briefly answered questions on his way through Sydney airport.

"All three of them were obviously sorry. They've seen the blowout that it's had back in Australia so, yeah, I can imagine that you would say sorry," Du Plessis said. "I don't think there's a person in the world that won't apologise for that.

"They've all accepted they're guilty, they've all put their hands up, they've all taken responsibility, or as far as I've heard in the press. So, for me, we all make mistakes. I'm certainly not perfect - I've made mistakes as well, but I've learned from them and they will learn from these mistakes and they'll be better for it."