Captain Brendon McCullum basked New Zealand's "phenomenal" success after their 27-run win - and unassailable two-nil series lead - over South Africa in the second ODI in Kimberley.

Entirely outgunned in the preceding Test series, which brought them back-to-back losses inside three days, the tourists struck back to trump an opposition five positions higher in the ICC rankings.

While Saturday's one-wicket victory was of a tight nature, Tuesday's triumph was completed with relative ease on the back of batsman Kane Williamson's fine century and the Proteas' dismal capitulation.

Williamson's 145 not out weighed in as the highest ODI score at the De Beers Diamond Oval, and carried the total to 279 for eight. South Africa's reply later reached dominant proportions, with half-centuries from opener Graeme Smith and the left-handed Colin Ingram steering the tally to 167 for one inside 31 overs.

The home side, however, lost their last nine wickets for a mere 85 runs, which included a record-equalling five run-outs. Spinner Nathan McCullum, meanwhile, recovered from an expensive opening spell to strangle the asking rate. He also had a hand in two run-outs and took an outstanding catch to get rid of the threatening Robin Peterson.

"It's a phenomenal win, the ninth-ranked team in the world against the top team. It'll go down as one of our biggest wins in the limited-overs games. I'm very pleased, and for the fans back home this is a bit of love for them as well," enthused McCullum.

"Williamson's innings was as good as we've seen from a New Zealander. Nathan was phenomenal in that spell of bowling and the fielding was probably as good as we have shown."

The right-handed Williamson was named Man-of-the-Match for the third ton of his career. Tuesday's vigil spanned 207 minutes and 13 balls, including 17 fours and a solitary six struck off the final ball of the innings. The latter stages of his knock were completed amid painful cramps.

"It was nice to contribute like that in such an important game. It's been tough playing the number one team in the world. I cramped up a little bit but the guys at the other end helped us get a good total," added Williamson.

The third and final match gets underway at Senwes Park in Potchefstroom on Friday.