New Zealand captain Forbes was sent to the sin bin for a deliberate knock-on early in the second half of their quarter-final, with South Africa taking advantage of the extra man from the restart to level the scores at 12-12.
But a touch of brilliance from Tomasi Cama created space for Tim Mikkelson to score a converted try. It was the score that sealed victory and brought relief fro Forbes as it cancelled out the South African try conceded in his absence.
As South Africa made one final desperate dash in the dying seconds, new player Ardie Savea managed a turnover at a ruck that would make any No7 in the XV-aside game proud. The 18-year-old had been introduced into the match as a substitute.
"I think 'Titch' [coach Gordon Tietjens] wanted me to do something" he said after the match. "And I just come on and do my bit.
"Just to get a win here and be in the final is a good achievement - it's a great achievement."
In the other semifinal, Fiji beat England 14-7 despite a muddled performance.
In their mission to defend their Hong Kong title, New Zealand had earlier powered past a hapless Wales in the quarter-final, hitting their straps to record a 31-7 victory.
The series leaders, on the hunt for an 11th success at Hong Kong and 10th overall series win, ran the show in their group matches. Both Wales (24-7) and South Africa (19-10) had already been beaten in pool play by the New Zealanders.
South Africa's path to the semifinal was tough, with an in-form Samoa presenting a physical challenge before the South Africans prevailed 27-14.
Wales advanced from the pools as one of the two best third-place teams despite only one win, over the United States.
Despite touches of rich form, Australia - who haven't won the Hong Kong title since 1988 - lost to England 19-14 in the quarter-final.
Traditional favourites with the Hong Kong crowd - where they have won a record 12 times - the Fijians kicked-off with a rout against Scotland then barely shifted into second gear against France, 26-12. In both those matches, Metuisela Talebula scoring two tries in both wins.
And even against their strongest group opponent, Australia, who had shown some blistering form on the pitch this weekend, the Fijians were too good and qualified at the top of Pool C.
- AP/Staff reporter