New Zealand sit second on the International Cricket Council rankings for the shortest version of the game, behind only world champions England.
South Africa are fourth, six points behind New Zealand, the two being separated by Sri Lanka.
McCullum, without excusing some of the shoddy bowling and fielding on show against Zimbabwe at Hamilton, suggested the idea of itching to get stuck into the South Africans might have been nestling at the back of players' minds.
"Sometimes the night before Christmas you don't always have the most settled sleep," McCullum said.
"We got a bit giddy in some of our options and executions and it wasn't our best game over the last few weeks, but I'm confident we can step up in the coming days and play at the level needed to beat South Africa."
McCullum knows the South Africans are the yardstick by which New Zealand's progress will be judged.
Zimbabwe was one thing - and had they lost one of the six international games, all manner of scorn would have been heaped on their heads - South Africa at the other end of the spectrum.
"If we want to continue to improve as an international team, become consistent and move up the rankings in all three forms, then you've got to beat the good teams and do it consistently.
"That's the challenge in front of us. But it's also an opportunity, so we're pretty excited."
McCullum pointed out New Zealand "are going to be tested in areas we haven't been over the last few weeks, but we're confident of being capable of stepping up and going toe to toe with them."
It's all good, rousing stuff but the only thing that matters is what happens on the park.
Left-arm fast bowler Lonwabo Tsotsobe took a hat-trick as South Africa beat Canterbury by 20 runs in last night's Twenty20 match in Christchurch.
Tsotsobe had opener George Worker caught by Morne Morkel for four with the fifth ball of his first over, bowled former New Zealand test batsman Peter Fulton for a duck next ball then trapped one-day international Shanan Stewart lbw with the first ball of his second over.
That reduced Canterbury to 9-3 in the fourth over in reply to South Africa's 20-over total of 150-6 and he went on to claim the wicket of current New Zealand batsman Tom Latham to finish with 4-9 from three overs.
Canterbury were all out for 130 to the final ball of the 19th over.