Mostert is drafted into the side to replace Flip van der Merwe, who was sent home with a rib injury suffered in the Springboks' 39-20 loss to Australia last Saturday.
The 26-year-old was only recently summoned into the squad just days after moving to Paris, where he will play his club rugby with Stade Francais.
"I was in Paris for five days, settling in, and I had most of my attention there getting to know the guys and then I got a call," Mostert said.
"It was a big surprise to me because I didn't expect it at all and I got on the first plane to get here."
Gio Aplon is another name to be added to a lengthy list of casualties, meaning Patrick Lambie - who normally plys his trade for the Sharks at fly-half - gets a first Test start at full-back.
Flanker Jean Deysel is also handed a starting berth for the first time, with Danie Rossouw shifting to number eight and Ashley Johnson dropped to the bench.
Wynand Oliver is another to lose his place in the starting XV after the loss to the Wallabies; Adi Jacobs starts at outside centre with Juan de Jongh moving into the No.12 jersey for the game in Wellington.
"We have had a few injury challenges on this tour and last week was not a good start, but we have learnt from last week's match," head coach Pieter de Villiers said in a statement.
"There have been some frank exchanges this week and the players know what is expected from them and that this is their opportunity."
South Africa listed 21 players as injured or unavailable for this year's Tri-Nations.
South Africa team: Patrick Lambie, Bjorn Basson, Adi Jacobs, Juan de Jongh, Lwazi Mvovo, Morne Steyn, Ruan Pienaar, Danie Rossouw, Jean Deysel, Deon Stegmann, Alistair Hargreaves, Gerhard Mostert, Werner Kruger, John Smit (captain), Dean Greyling.Reserves: Chiliboy Ralepelle, CJ van der Linde, Ryan Kankowski, Ashley Johnson, Charl McLeod, Wynand Olivier, Odwa Ndungane.
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