But with Rob Nicol coming in as opener for his debut at Dunedin, McCullum eased down one spot and his double of 48 and an unbeaten 58 impressed.
McCullum's numbers at No 3 are eye-catching, a test average of 51, albeit after just seven innings.
McCullum and captain Ross Taylor had shared an unbeaten, impressive third-wicket stand of 82 to have New Zealand well positioned for a final-day push at Dunedin before the rain ruined a fascinating situation.
McCullum may prefer opening, but No 3 is a pivotal job. Taylor confirmed McCullum will remain there for this test, with the top five in the batting order to occupy the same positions.
A fresh challenge in a critical role should get McCullum's competitive juices flowing.
"If we wanted Brendon to open he would have in Dunedin. At this stage, we're pretty happy the way things are going," Taylor said yesterday.
The main selection issue for Taylor and coach John Wright yesterday was the makeup of the bowling attack.
Although they had a fair idea of the final XI last night, no players had been dropped out of the 14.
It is unlikely leg spinner Tarun Nethula will get his first test, and at least one of Brent Arnel or Mark Gillespie, aged 33 and 32 respectively, and with five and three tests behind them, will receive a recall.
"Arnel knows this wicket pretty well. He's been the form bowler, bowls wicket to wicket and can hurry players up," Taylor said of the Northern Districts seamer.
Stocky Wellington quick Gillespie was "aggressive. He's probably not going to go at two or three an over but he will pick up wickets".
Taylor said since rejoining the squad "he's the normal 'Dizz', he talks a big game. It's good to have him around".
South Africa had an injury niggle, batsman AB de Villiers having damaged an ankle in training on Tuesday. JP Duminy is the likely replacement if de Villiers doesn't make it.
The world's No 1-ranked test bowler, Dale Steyn, has had a toe problem, but captain Graeme Smith allayed concerns.
"If I look around the fast bowlers in the room, they've all got pretty horrific-looking toes, so it's something that comes with the job. Dale says he's 100 per cent ready to go. He's really hungry to do well in this test."
Twelve of Seddon Park's 18 tests since 1991 have produced results, New Zealand winning seven and losing five. The past five have all had an outright result.
The pitch has steadily browned off this week but both captains said bowling first will be a consideration before the toss.
NZ V SOUTH AFRICA
Seddon Park, 10.30am today
New Zealand (from): Ross Taylor (c), Martin Guptill, Rob Nicol, Brendon McCullum, Kane Williamson, Dan Vettori, Kruger van Wyk, Andrew Ellis, Doug Bracewell, Tarun Nethula, Trent Boult, Brent Arnel, Mark Gillespie, Chris Martin.
South Africa (probable): Graeme Smith (c), Alviro Petersen, Hashim Amla, Jacques Kallis, AB de Villiers, Jacques Rudolph, Mark Boucher, Vernon Philander, Dale Steyn, Morne Morkel, Imran Tahir.
Umpires: Richard Kettleborough (England), Billy Doctrove (West Indies). Third umpire: Aleem Dar (Pakistan).
