The match had begun terribly for McCullum, who lost his 17th toss in 28 tests as captain.
His team was sent in to bat and his counterpart Angelo Mathews could barely suppress his glee at the amount of grass cover still left on the pitch.
Martin Guptill and Tom Latham, however, blunted the Sri Lankan attack and raced along at more than four runs an over in an opening stand of 56 that set the tone.
"The way Martin Guptill and Tom Latham started, it was going to be tricky that first hour or two and ... the guys played exceptionally well," McCullum said.
"They hit the ball down the ground and every time the Sri Lankans missed they were able to capitalise and we got off to a four-an-over start and in a test match that's not easy to do.
"We gathered momentum from there and to put on 400 on the first day, yeah, we probably lost a couple more wickets we would have liked but it advanced the game."
New Zealand were 409-8 at stumps on day one, with time to bowl the tourists out twice on a wicket not expected to deteriorate quickly.
Sri Lanka proved defiant in their batting and it took New Zealand 117 overs to bowl them out for 294 in their first innings.
McCullum surprised many on Sunday with a declaration that set a target of 405 to win, when a total above 450 might have been safer.
"Sometimes you have to risk losing to win as well and we felt that we needed enough time to bowl Sri Lanka out," McCullum said.
Rain and hail showers, which forced players off the field three times on Sunday, also played a part.
"If the weather had come into play a bit more today we would have desperately needed that time," he added.
"But when you have guys like Neil Wagner who is going to come in and bowl long and hostile spells leading up to a new ball then you know you're going to be in the game.
"In the end you have to back your bowlers and I think the way they went about it was outstanding."
(Reporting by Greg Stutchbury in Wellington; Editing by Ian Ransom)
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