Postecoglou coached eight winless games at Nottingham and was sacked after 39 days, the second-shortest managerial tenure in English Premier League history.
His Forest stint was a steep fall from grace following his two seasons with Tottenham Hotspur, where he ended a 17-year trophy drought with a 1-0 win over Manchester United in the UEFA Europa League final in May last year.
Tottenham sacked Postecoglou the next month, former executive chairman Daniel Levy citing the drop from fifth to 17th in the league, despite suffering 41 recorded injury cases and a shift to prioritise key players for European competition.
In September, the 60-year-old replaced Forest's Portuguese manager Nuno Espirito Santo, who had fallen out with club's owner Evangelos Marinakis.
The Greek multi-billionaire axed Postecoglou 19 minutes after the full-time whistle rang out to boos at Forests' City Ground after an unceremonious 3-0 defeat to Chelsea on October 18.
"That was a bad decision by me to go in there and I've got to take ownership," Postecoglou told The Overlap podcast.
"There's no point in me blaming (Forest) that I didn't get time.
"I should never have gone in there."
Both the Trees and Spurs have subsequently sacked Postecoglou's successor.
Thomas Frank exited Tottenham with the club 16th in the Premier League, though they finished fourth in the UEFA Champions League league stage.
Hours after Frank was sacked, Forest parted with Sean Dyche to mark Marinakis' third sacking of the 2025-26 season, with the club one spot below Spurs in 17th.
Postecoglou's attacking style of play earned him plaudits at Spurs but proved ill-fated with Forest, who had achieved sixth place the previous season with a more pragmatic counter-attacking system.
"It was too soon after Tottenham," Postecoglou said.
"I'm going to do things differently (than) they were used to.
"I've got to cop that; that was my mistake."
Former Arsenal striker Ian Wright asked Postecoglou if had been advised to not take the Forest job.
"Everyone around me was saying that," the former Socceroos manager said.
"It was the first time I wasn't working in 20-odd years and I was lost."
Gary Neville asked Postecoglou his opinion on what Manchester United should do when the head coach role becomes vacant at the start of next season.
Postecoglou labelled it the "hardest job in world football" due to the scrutiny on any successor of legendary former United manager Sir Alex Ferguson.
He listed several traits the next head coach would require, resembling the principles that made him fabled at Spurs.
"Winning and not winning in the right style," Postecoglou said.
"History being a constant companion to everything you do.
"Someone who will say: 'I will do it, but I'll do it my way'."
Postecoglou said he was in discussions about a return to the media for the upcoming FIFA World Cup, though his next club, or any indication of his closeness to the role of Manchester United head coach, remained unclear.
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