Rennie revealed he already has a team pencilled in for next month's first of four Tests against the world champion Springboks, dubbed the "Greatest Rivalry" series.
But classy fly-half Mo'unga won't feature when the coach names a 44-man squad for the high-profile tour, he told journalists Thursday.
Mo'unga was New Zealand's play-maker at the last two World Cups but under the national union's strict eligibility rules, has been out of contention since leaving to play three seasons of club rugby in Japan in 2023.
The 32-year-old has returned home and signed a New Zealand contract through to next year's World Cup but he remains ineligible until October, when he completes a season in the domestic provincial competition.
There had been speculation Rennie would try to force a fast-tracking of Mo'unga's return for the South African series.
However, the coach said he had consigned himself to accepting the status quo, although he pointed out Mo'unga can be called up for the tour if another fly-half gets injured.
"He won't be coming on that tour, so we'll squash that," Rennie said.
"If a 10 got injured, then Richie's an option for us to bring in.
"But for now he'll just get back and play and get himself in great nick so if he gets an opportunity to come to Africa, he's ready to go."
Rennie said he needed no proof that Mo'unga was ready for an international return, having watched him while coaching a rival club in the Japanese league.
"I mean, I got to see him first hand in Japan in the last three years where he's been phenomenal."
New Zealand will play against domestic South African sides the Stormers, Sharks and Bulls before the opening Test in Johannesburg on August 22.
Rennie said every player will be given an early chance to push for Test selection but revealed the selectors already had a preferred team in mind.
"We've talked about lots of things. We've picked the first four squads for the first four games in Africa," he said.
"So we've had a lot of thought around where we want to see guys.
"Everyone will get to play in that first week, so it's an opportunity for everyone to push for places."