"It really hit home to me that it was quite a significant achievement for a New Zealander, and I'll certainly remember it for the rest of my life."
A tear in the eye, perhaps?
"Nah, no tear. I'm from south Dunedin," he quipped.
He'd had a beer with father, former first class player Stu, on Monday night, and would have one with his teammates last night, when they would "just reflect on a season that's been pretty important for us and the game in New Zealand".
New Zealand batted until just before lunch, once Jimmy Neesham had reached an outstanding century on debut, 137 not out.
Briefly there was a glimmer of something dramatic as India, chasing 435, slipped to 54 for three, but Virat Kohli - who dodged an umpiring blunder on 15 when caught at the wicket - hit a brilliant century, which will be forgotten amid the more significant events of this test. The match was called off 15 overs early.
McCullum wasn't about to allow India back into the contest, hence no declaration once his 300 had been reached.
"Two and a half days ago we were staring down the barrel of an early finish on day three," he said.
"We didn't want to pass up all that good work we'd done to try and give India a sniff back into the series."
Quite right too.
Remember the flak McCullum took over a decision not to enforce the follow-on in the first test at Eden Park and think of the reaction had it gone wrong.
So what if he'd followed his attacking instincts and left India, say, 380 to win and the Indian batsmen went on a tear?
It's not worth pondering. There are times when safety comes first and this was one of them.
McCullum watched Martin Crowe's 299 in 1991. He heard Crowe talk about what it would mean on TV yesterday, and spoke to Stephen Fleming, second on that highest individual score list on Monday night.
"I feel a little bit embarrassed because I'm nowhere near the calibre of players they were."
McCullum thought of the tough times - and he's had his share, even as recently as four months ago - and has discovered the value of perseverance if there's a strong belief you're on the right track.
New Zealand rose to seventh in the world rankings last night. Sri Lanka, in sixth, are within reach.
But they are entitled to put their feet up and savour a remarkable day.