A double to Sherwin Stowers and a runaway effort to Tim Mikkelson saw the home side rack up all their points in the first spell and then they relied on their impressive defence to close the game.
After dropping their opening pool game 12-7 to Fiji yesterday, New Zealand didn't concede a point during their next five outings and their victory in the capital also meant they jumped back to the top of the world series standings after five rounds.
New Zealand had lost the past two tournament deciders to the South Africans so this victory will taste extra sweet as the defending world series champions put everyone else on notice.
Mikkelson was impressive during the final as was Bryce Heem, who stood out all tournament, while Stowers was a constant threat in the backline.
Heem had a hand in two of New Zealand's tries, while a crucial lineout steal from Scott Curry led to Stowers' first.
With a comfortable lead at the split, New Zealand kept their line intact as South Africa failed to come up with an answer.
New Zealand 21 (Sherwin Stowers 2, Tim Mikkelson tries; Gillies Kaka 3 con) South Africa 0 Halftime: 21-0.
