Prop Sika Pole had his marching orders after 23 minutes for a clumsy high shot on Junior Boks loose forward JJ Theron, initially given a yellow card that was swiftly upgraded to red in Rovigo on Saturday (local time).
Even though it was a 20-minute red card under tournament rules, and the South Africans found themselves down to 14 just before halftime with a yellow card, it proved to be a real turning point in the game.
In New Zealand's first final in seven years, Pole leaving the field meant that he had to be replaced by another front rower, so influential loose forward Mosese Bason had to head off too for the 20-minute period.
The difference in doctrine between the two sides was evident, as the New Zealanders seemed intent to run the ball while the South Africans favoured a heavy kicking game.
Jayden Sa struck back for the Kiwis with a slick try off a sneaky lineout move after 16 minutes, before Moyo kicked a couple of penalties to make the score 13-5 at halftime.
A very long period of attack by New Zealand after the break saw them batter the Junior Boks' line, but they couldn't crack it and had to settle for a Rico Simpson penalty.
Moyo cancelled that out with a long range penalty off a scrum penalty after 56 minutes.
The Junior Boks' scrum proved to be the decisive factor in the second half, as the big South Africans dominated the New Zealanders.
Halfback Haashim Pead looked to have scored off the back of a big shove but it was disallowed, however by then it was obvious New Zealand would need something special after being pinned in their own half.
They had one chance after an excellent turnover by Eli Oudenryn with 12 minutes to go, but the replacement hooker went from hero to zero at the subsequent attacking lineout with a crooked throw and then getting penalised for backchatting to the referee.
Another yellow card, to lock Xavier Treacy, and a late try to Junior Bok fullback Gilermo Mentoe sealed their fate as time ran out.
The Kiwis managed to grab a consolation try in a frantic last couple of minutes - but it was too little, too late,
The Junior Boks secured their first World Championship win since 2012, snapping a Northern Hemisphere grip on the title dating back to 2018.
