The world champions still have two Tests left in their 2025 Nations Series campaign with the eagerly anticipated Test against Ireland at the Aviva Stadium on Saturday followed by a clash with Wales at the Principality Stadium in Cardiff on November 29.
Double Rugby World Cup winner Mbonambi was part of the Springboks' Rugby Championship-winning squad earlier this year and was one of the players named on Erasmus' standby list for the tour.
Meanwhile, Mchunu, who recently moved from the Sharks to the Stormers, has made three Test appearances for South Africa and won his last cap against Portugal in Bloemfontein during the 2024 mid-year internationals.
"This is a longer tour than usual, and we have two big matches lined up against Ireland and Wales, which prompted the decision to call up Bongi and Ntuthuko," said Erasmus.
"Both players have done the job for us on the international stage, so we are excited to have them in the group.
"Several players will not be eligible for selection for the final Test on tour against Wales, as the match falls outside of the international window, so it makes sense to call up the players now to get back into the swing of things with us at training this week, while at the same time increasing the depth we have within the squad for our next challenge against Ireland."
South Africa clinched a hard-fought victory over Italy in Saturday's Nations Series encounter in Turin and took a gamble in that match by heading into the encounter without a recognised hooker on their bench.
Johan Grobbelaar wore the number two jersey but props Gerhard Steenekamp and Wilco Louw were the only front-row options amongst the replacements with Marco van Staden, who started at number eight, covering the hooker position.
That gamble nearly backfired as the world champions had to play with 14 men for most of the match after experienced second-row Franco Mostert was red carded for a dangerous tackle on Paolo Garbisi in the 12th minute.
Fuming Rassie Erasmus hits out at 'not equal' officiating of head shots after second successive Springboks red card
Despite that setback, they fought back and although both sides received further yellow cards, the Boks eventually prevailed as victors by a 32-14 margin.
Speaking about how the team have an immense ability to adapt to such situations on the field, Boks skipper Kolisi said: "We went through it last weekend, and this week we went through the same thing.
"So, we make plans in the game as we go along because such things happen.
"The biggest thing that we were asked this week is to show our 'Africanness' and that's about making plans because there are always stumbles, and that's what I am grateful for with our coaching staff.
"They are always prepared for any scenario, and even the guys who are not playing sit and make plans and decisions, they always buy in."
Kolisi added: "It doesn't make it easy for us. We can talk about sacrifices, but seeing a guy like Franco sitting there, we could see the hurt in his eyes.
"That said, I really love the way this team is just able to stand up and fight. We always say whatever happens between those four lines, we can still go as hard as we can and control what we can."
The Springboks will travel from Italy to Dublin on Sunday afternoon and will begin their on-field preparations for the match on Monday.
Springbok lock RG Snyman is expected to earn his 50th Test cap on Saturday when South Africa face Ireland at the Aviva Stadium, a milestone that will come in the country where he has spent most of his club career since 2019.
The Springboks will host England, Scotland, and Wales next year as the southern and northern hemispheres embark on the groundbreaking new Nations Championship.
Damian Willemse has revealed that talk of South Africa sending a second-string team to face Italy fired up the players, calling the narrative "disrespectful" after their hard-fought victory in Turin.
England have moved up to third place in the latest World Rugby rankings, ahead of Ireland who also won over the weekend.
