The Kiwi mentor was confirmed as the next All Blacks coach on Wednesday, replacing the sacked Scott Robertson and beating Highlanders coach Jamie Joseph to the position he'll hold until at least the end of next year.
He will become the first man to coach both the Wallabies and the All Blacks.
It's an intriguing appointment given Rennie was due to take the Wallabies into the 2023 World Cup in France, before Eddie Jones was sensationally reinstated at his expense.
Jones drastically overhauled the squad and the Wallabies bombed out, failing to escape the group stage for the first time in Cup history.
The coach lasted less than a year into a five-year deal and is now coaching Japan's national team, where Rennie is based with League One outfit Kobe.
Taking charge during the COVID-19 pandemic, Rennie won 13 of his 34 Tests at the helm (38 per cent) to finish with the lowest winning percentage of any Wallabies coach to oversee more than 30 games.
There were green shoots though, with a first-up draw against the All Blacks, and a 3-1 record against the Springboks.
The Wallabies were gutted by a one-point loss to France in Paris that ultimately contributed to his axing when they also lost to Italy on an injury-hit 2022 Spring Tour.
Rennie resisted temptation to bite during or after Jones' eventful Wallabies tenure and on Wednesday reflected with dignity.
"I've got a lot of strong relationships there. I learnt a lot, coaching at that level," he said of his time with the Wallabies.
"It wasn't an ideal end and I obviously didn't get the chance to go to a World Cup, but I felt more for probably about 13 or 14 players who would have gone to a World Cup with me, who didn't get to go either.
"So yeah, I'm looking forward to the opportunity to go to one next year."
Rennie, who won back-to-back Super Rugby titles with the Chiefs in his last New Zealand posting, will return home once the Japanese season concludes in June.
His first Tests in charge will be a home series against France, Italy and Ireland in July, while the Wallabies await in an Auckland Bledisloe Cup opener on October 10, one year before they lock horns in a World Cup pool fixture.
The task will be to settle a squad that appeared to fracture during a 2025 campaign under Robertson that included a record defeat to the Springboks and first loss to the Pumas in Argentina.
Players were interviewed as part of a season review, with Ardie Savea reportedly among the most critical of his methods.
The Moana Pasifika star is on sabbatical at Kobe, under Rennie, and due back next year ahead of the World Cup.
Playmaker Richie Mo'unga is due to return from Japan later this year while star lock Brodie Retallick could be coaxed back home after impressing Rennie at Kobe this year.
"I'm not going to divulge what conversation we've had, but he's a proud All Black," Rennie said of Savea.
"He's playing good footy for us. I know he's really excited about the future, our job is to make sure he returns mentally refreshed but also in better condition than he arrived.
"If you want to win a World Cup ideally you've got (to have) your best players available.
"Richie's coming back ... (and) someone like a Brodie Retallick coming into the environment, I reckon it will really grow the whole group. He's done a phenomenal job around leadership at Kobe."
