Kohli accused Smith of abusing the decision review system (DRS), after he was seen looking to the Australian dressing room for advice on whether to ask for a review when he was being given out lbw.
Smith described his breach as a "brain-fade".
Du Plessis has always denied ball tampering and said the reaction from the ICC was different to the way he was treated.
"Definitely, it was different," he said. "I'm surprised by the way it happened.
"Maybe it's because I speak on a personal point of view and felt I was treated very harsh. When you see something like that, you'd hope it [the treatment] would be exactly the same."
Du Plessis said it was very unlikely there would be a similar dust-up between the South African and New Zealand players.
"When you play teams like India and Australia, obviously that can happen and it's easy for something like that to blow up," he said.
"For me, it's just been good to be on the other side of it this time and see how things unfold."
The first test between New Zealand and South Africa ended in a draw, when rain prevented any chance of play on the final day, with South Africa 224 for six in their second innings, leading New Zealand by 191.
The second test starts in Wellington on Thursday.
Meanwhile, wicketkeeper Matthew Wade won't be afraid to fire some barbs back at India in Ranchi, admitting the hosts' antagonist approach had come as somewhat of a shock in the second test.
India were largely quiet and courteous in the field during the four-test series opener in Pune, where Australia recorded a shock victory.
Kolhi urged team-mates to be far louder and more lippy in Bangalore. The captain led the way, making a throat-slitting gesture after the fall of one wicket and aggressively swearing at most batsmen.
Whether co-incidence or correlation, India levelled the series. Wade noted, when Australia returned to training for the first time since the 75-run loss, that "emotion doesn't win test matches".
But the chirpy keeper has no qualms about trying to match fire with fire in the clash that starts on Thursday, should it help Australia's push for a 2-1 series lead and retention of the Border-Gavaskar trophy.
"I feel like I play my best cricket when I get in the contest ... that's a big part of my game," Wade said.
"Getting older, I probably tamed it down a little bit, and I probably know when to use it a little bit more now and when I need it myself.
"If there's a time I think it can be an advantage for us, sure, I'll go for it. "If it's needed, I'll definitely use it, but we've got to play them on skill, not emotion."
On Wednesday, Kohli and Steve Smith will meet with incoming match referee Richie Richardson, who has been put in an awkward position by his employers.
Richardson must convince Kohli and Smith to change their sides' behaviour, despite the International Cricket Council clearing all 22 players involved in one of the most-heated Tests of the modern era.
"They're always aggressive. The change from the first test to the second test was probably the initial shock," Wade said, likening India to a "caged lion".
"They came out a lot harder. We expect it for the rest of the tour.
"Our job is to make sure they can't get in the game, so they can't get aggressive with us."
