The Black Caps opted to bowl at the toss and quickly put the brakes on the Proteas scoring as they only reached 34 after the powerplay for the loss of captain Rassie van der Dussen (14).
The South Africans continued to lose wickets at regular intervals while the scoring-rate did not improve much, Dewald Brevis (13) briefly threatened but was caught behind for off lanky paceman Will O'Rourke (1-26).
Opener Reeza Hendricks struggled to find his timing on a difficult pitch to bat on but remained at the crease until the 17th over before being dismissed for 41 from 37 balls.
George Linde struck an unbeaten 23 off 15 balls to push the Proteas to a total of 134-8 in their 20 overs with seamers Jacob Duffy, Adam Milne and skipper Mitchell Santner taking two wickets apiece for the Kiwis.
New Zealand got off to a quick start in the run-chase after openers Tim Seifert and Devon Conway (19) put on 51 in five overs before the latter was dismissed by seamer Andile Simelane.
Senuran Muthusamy was again good in his four overs, the left-arm spinner taking 2-24, but Seifert was unbeaten on 66 and Daryl Mitchell 20 not out as New Zealand reached the target in the 16th over.
Proteas head coach Shukri Conrad has confirmed that veteran opener Reeza Hendricks has fallen out of favour in the national T20I setup, with several batters now ahead of him in the selection hierarchy.
New Zealand produced a clinical performance to defeat Zimbabwe by 60 runs, recording the highest total of the tri-series and sealing their place in Saturday's final against South Africa.
Proteas Men's head coach Shukri Conrad has announced two full-strength squads for August's white-ball tour against Australia Down Under.
The Proteas advanced to the final of the T20I tri-series in Harare with a seven-wicket victory over hosts Zimbabwe on Sunday, a result that also eliminated Zimbabwe from contention.
