Australia captain Pat Cummins has bowled in-form India batter KL Rahul with the final ball before tea on day two of the Boxing Day Test.

Rahul (24) and Yashasvi Jaiswal (23no), India's leading scorers during this Border-Gavaskar series, had been batting superbly as they set about eating into Australia's first innings 474.

But Cummins (2-27) struck with a trademark delivery, hitting the top of Rahul's off-stump to leave India 2-51, trailing by 423.

After opening together in the first three Tests, Rahul shifted to No.3 in Melbourne to accommodate captain Rohit Sharma at the top with Jaiswal.

But a move back to opener failed to help Rohit out of his form slump, casting further doubt on his Test future.

Jaiswal and Rahul are the only members of India's top-six to score more than 150 runs in this series, with Virat Kohli next on 126.

The 37-year-old Sharma fell to rival captain Cummins after an ugly, mis-timed shot to the leg-side, prompting fierce criticism from former stars in the commentary boxes.

Earlier, Steve Smith's late-career revival continued with the star veteran posting his 34th Test century to power Australia to 474.

After starting day two at 6-311, Australia added another 4-163 on Friday before being bowled out just after lunch.

Smith brought up tons in back-to-back Tests when he drove Nitish Kumar Reddy for four midway through the first session.

His innings ended on 140 off the seventh ball of the second session when he played on to his stumps against Akash Deep.

India piled on 6-487 in the second innings of their huge win in Perth, but have only made one other score exceeding 200 in the series.

Cummins (49) also added valuable runs, sharing a 112-run seventh-wicket partnership with Smith.

The 35-year-old Smith arrived at the MCG after scoring his first Test century since June 2023 when he hit 101 in the rained-out match at the Gabba.

It was Smith's 11th Test ton against India, adding to his 12 made in Ashes battles.

Smith overtook England star Joe Root (10) for most Test centuries scored against India.

It was also Smith's fifth Test hundred at the MCG, a venue where he has 1209 runs at an average of more than 86.

The former Australia captain seemed to be facing his cricketing mortality after making a combined 19 runs in the first two Tests of the series, which extended his Test century drought to 18 months.

Continuing on from an unpredictable start to the Test, play was briefly interrupted on Friday morning when a pitch invader wearing an Indian shirt ran onto the field and touched Rohit and Virat Kohli.

Security eventually got to the man, but not before Kohli was forced to awkwardly interact with him.

Day two came after teenager Sam Konstas smashed an unforgettable 60 to begin his international career on Thursday, bravely taking on India's strike weapon Jasprit Bumrah (4-99).

Bumrah's 99 runs, from 28.4 overs, is the most he's conceded in an innings during his glittering 44-Test career.

Usman Khawaja (57) and Marnus Labuschagne (72) also made starts, making it the first time Australia's top-four had all passed 50 in the same innings since the first Test in Pakistan in March 2022.

Cummins won his first toss of the series, electing to bat first to escape the sweltering conditions on Thursday when the temperature soared to 39C.

The MCG has become a bowler-friendly venue since Scott Boland's famous debut against England in 2021.

He will be looking to recapture some of that magic with the ball as Australia attempt to go 2-1 up in the series.

India would retain the Border-Gavaskar trophy if they pull off a comeback win in Melbourne.

Australia need to prevent India from winning one of the last two Tests in this series, while winning at least one themselves, to regain the trophy for the first time since 2014-15.