India batter KL Rahul's contentious caught behind dismissal in the opening Test against Australia kicked up a controversy with former players from both the countries questioning the third umpire's decision to overturn the on-field official's not out call.

After on-field umpire Richard Kettleborough ruled in Rahul's favour following Australia's appeal, the home team used DRS to challenge the decision.

Third umpire Richard Illingworth overturned the call despite not having the benefit of a split-screen view which would have given him a clearer picture of whether the Mitchell Starc delivery actually grazed the bat or the snicko responded to a hit on the pads.

All this unfolded 10 minutes before lunch and India ended the session at 51 for 4. Rahul, who made 26 off 74 balls, indicated that his bat hit the pad at the same time when the ball went past the edge.

"I've got a spike when the ball passed his outside edge," Illingworth was heard saying while deciding on the DRS appeal.

Rahul shook his head in frustration while walking off the pitch.

Former India head coach Ravi Shastri, who was commentating for Fox Cricket, said there was not enough evidence for the third umpire to overturn the on-field official's not out decision.

"My initial reaction was, was there enough evidence there for the third umpire to overrule what was given. It was not out on the field of play. Did I see enough there for me to be convinced? I didn't see enough, to be honest," he said.

Former Australia batter Michael Hussey admitted on air that the third umpire's decision was a controversial one.

"That's controversial - there was a spike on the Snicko, but was the spike coming from the ball hitting the bat, or was it the bat hitting his pad?" Hussey asked while commentating for the same channel.

"You can see the bat just clipping the pad, so you've just got to get the timing right...There's got to be some doubt there in my mind."

According to Hussey, Rahul had every right to question the decision, saying: "I don't think you can be 100 per cent sure that the decision is correct."

"The disappointing thing is the technology's there to make sure you get the correct," he said.

Former Australia opener Matthew Hayden also felt that the spike in the snicko was not because of the ball taking an edge from Rahul's bat.

"His (Rahul's) pad and bat are not together at that point in time as the ball passes. It (bat hitting pad) is after, in fact, the ball passes the edge," Hayden said on air.

"Does Snicko pick up the sound of the bat hitting the pad? We're assuming (Snicko) may be the outside edge of the bat but that may not be the case."

Australia batting legend Mark Waugh added: "That's a very brave decision given the evidence that we've seen there; unfortunately KL Rahul's got to cop it sweet … (he) won't be happy with the way it's ended."

Former India players Wasim Jaffer and Irfan Pathan also felt the third umpire erred in his decision.

"Third umpire asked for another angle which wasn't provided. I'd assume he'd only ask for another angle if he wasn't sure. Then if he wasn't sure, why did he overturn the on field not out call?

"Poor use of technology and proper protocol not followed. KL hard done by," Jaffer tweeted.

"If it's not clear don't give it out!" Pathan wrote on 'X'.

Former ICC elite umpire Simon Taufel also felt that Rahul wa per haps unlucky.

"We saw with that side on shot there was a spike on RTS with the bat away from the pad, in other words the bottom of the bat hadn't reached the pad," he was quoted as saying by '7Cricket'.

"Therefore rolling that through in its natural course, you may have seen that second spike (on Snicko, to indicate bat hitting pad) come through, had it been rolled all the way through."