The New Zealand attack, after taking early wickets to reduce South Africa to 3-35 inside 10 overs seemed powerless to stop the Proteas middle order as they chased down the hosts' 9-253 with ease.
Doug Bracewell, who has featured in all of the internationals this season, and Tim Southee, who missed only the first Twenty20 match against Zimbabwe, both went at more than six an over and the pair appear prime candidates for a break.
Left-arm pacemen Andy McKay and Michael Bates as well as legspinner Tarun Nethula may get a crack on the traditionally batsman-friendly Hawke's Bay pitch.
Regardless of the side that takes the field, McCullum knows they have to be on song to beat the confident tourists, who have now won three matches in a row.
"They're a very good team, we know that, but they are playing in our conditions and we should have put up a better fight than what we did,'' he said.
Proteas skipper AB de Villiers, whose classy unbeaten 106 guided them home with 28 balls to spare, is looking to wrap up the series with a game to spare.
"Momentum is on our side and we would like to go two-nil up and win the series,'' he said.
- AAP