The midfielder has earned two man-of-the-match awards for his performances in the tournament opener against Kuwait and Tuesday's 2-0 semi-final win over the United Arab Emirates.
Asked on Wednesday whether he thought the 22-year-old would be as good as he's proven this campaign, Postecoglou said: "No, probably not."
"He's played every game and if you'd had have said to me at the start of the tournament whether he was going to, I would've very much doubted it," he told Sky Sports Radio.
"We've tried to rotate most of the squad because we felt it was going to be a pretty demanding schedule.
"But it's been hard to leave him out, he's been fantastic in every game.
"It seems like every game he's becoming more and more confident. He's been a fantastic bonus for us."
Luongo, contracted to Swindon Town in England's third tier, was part of Australia's World Cup squad but didn't play.
He said post-match on Tuesday he was a little surprised by his achievements.
"I know within myself what I'm capable of," he said.
"To be fair, it has only been tested at (English) League One level, so to bring it out on the big stage like this is a little bit surprising.
"But I feel like that I'm getting away with a lot. Maybe I should be believing in myself a little bit more.
"And maybe I can do a little bit more in the next game."
