New Zealand stormed into the ICC T20 World Cup final with a 9-wicket demolition of South Africa thanks to Finn Allen's record-breaking century.

Allen's unbeaten 100 off 33 balls was the fastest in T20 World Cup history, eclipsing Chris Gayle's 47-ball ton.

The opener's explosive knock, paired with Tim Seifert's 58, made quick work of South Africa's 170-run target.

Marco Jansen's 55* off 30 balls was not enough to guide the Proteas to a defendable score.

New Zealand's bowlers, led by Cole McConchie, Rachin Ravindra and Matt Henry restricted South Africa to 169-8.

South Africa's ICC knockout struggles continue, with captain Aiden Markram acknowledging that Allen's innings was a crucial factor in the result.

"When you're on the receiving end of an innings like that, you don't often come out with the right result," he said post-match.

New Zealand's victory will give them real belief that their hunt for a first ever World Cup title could come to an end -- with just one game separating the Kiwis from silverware.

The Black Caps will face the winner of India versus England in Sunday's final at the Narendra Modi Stadium in Ahmedabad.