There was a 6-4-4-6-1 sequence off Marsh shortly after.
The buccaneering McCullum had 29 off his first 11 balls; took 22 in seven Jackson Bird deliveries; another 14 in four Josh Hazlewood balls before going from 82 to 100 like this: 6-4-4-4 off Hazlewood, the last of them charging down the pitch to drive to the extra cover boundary.
It was the 54th ball he had faced and broke the record for fastest test ton by two balls.
The final lap from 82 to 100 caught the crowd off guard. It wasn't until the last two boundaries that a buzz grew around the ground when word filtered out that the world record was on.
''I was just trying to hit every ball for four or six,'' he reflected.
It's unlikely Cook in his entire career would have taken McCullum's approach on 96.
McCullum's final innings a couple of days later produced 25 off 27 balls, his second last ball swatted into the crowd at square leg, before falling to a flying catch by David Warner at mid wicket.
The sign-off centuries Cook and McCullum mirrored the way the men played their cricket.
No lefthander has scored more runs in tests than Cook; few batsmen have batted with such electricity in his arms, and such fearless bravado, as McCullum.
Only four men have previously scored centuries in their first and last tests — the most recent, former Indian captain Mohammad Azharuddin in 2000 — with the first three being Australians, Reg Duff, Bill Ponsford and Greg Chappell. Cook now joins that elite group.
McCullum's first century didn't come until his seventh test.
But his Hagley Oval heroics enabled him to leave New Zealand cricket in an appropriate way. The same could be said of Cook.