However McCullum is confident the top order is looking sound ahead of the tests, and the fast-bowling challenge posed by premier South African quicks Dale Steyn and Morne Morkel.
McCullum and in-form Martin Guptill will open in the first test, with Kane Williamson and returningt skipper Ross Taylor expected to be at Nos 3 and 4.
Taylor will get a chance to test his calf muscle when Central Districts play Auckland in their Plunket Shield match starting on Friday. His last test innings was a century against Zimbabwe in Napier on January 26, the day before injured himself.
''I think the top order is ticking along," McCullum said.
''We're standing up and getting ourselves in pretty good situations and then folding through the middle."
The challenge will only get stiffer in the five-day contests when bowlers have more opportunity to keep a squeeze on the batsmen.
''We're going to be tested a lot more in the tests and with their love of attacking us short," he said. ''So that's going to be a good eye-opener and to see how we respond to that."
South Africa rate themselves in a rebuilding phase, which should be a worry for their future opponents.
''Obviously they've got good firepower with the ball and experience with the bat," McCullum said.
''Their fielding as always is very good and they use that as an attacking weapon, so while they are rebuilding they are doing it from a position of experience. We're certainly not surprised with how well they've played. We've got to look at ways we can get closer to them and hopefully respond better."
First though is the final ODI. De Villiers suggested before the Napier win that changes may be made if the series had been decided to ensure those not chosen get a run before the tests.
That could point to young quick Marchant de Lange getting a game at Eden Park, although fast bowling is one area South Africa are particularly strong.
Apart from Steyn and Morkel, left armer Lonwabo Tsotsobe has done a good job with the new ball and fast-rising Vernon Philander is arriving in time for the tests.
When South Africa won the first three ODIs of their last series at home to Sri Lanka, de Villiers wasn't impressed with losing the final two games of that rubber.
''We're still a young side and we'd like to keep that momentum. I'm chuffed so far but the work is not done yet," he said.
The New Zealand squad of 14 for the Dunedin test, which starts on Wednesday, is due to be named on Friday.