After being sacked by Tottenham at the end of last season, Postecoglou lasted just eight games before his dismissal by Nottingham Forest on Saturday.
Agbonlahor told talkSPORT: “Yeah, Ange is 100 per cent done.
"Everyone would have been shocked that he actually got the Forest job after what happened at Spurs in the Premier League, and to get the sack after five Premier League games, shortest managerial run in Premier League history.
“I think if Ange gets a job he will be abroad, maybe an international team.
"But I'm sure his family and his young kids are still in Australia, he can go back home now, enjoy some family time, because the stress of his last two jobs can't be good for him."
Asked if Postecoglou is a good coach, Agbonlahor also said: “I don't think so.
“Listen, he did great things at Celtic, fantastic winning the Europa League at Spurs, but 22 Premier League defeats, eight without a win before he got sacked at Nottingham Forest.
“It doesn't look right - the change of formation. Didn't name the same team twice in eight games, which is wild."