Postecoglou is convinced an insider is giving team information consistently to the media.
He said on Friday: "There is no doubt we've got a leak inside the club. Someone continues to leak out information and they have all year.
"It started last year. We thought we had nipped it in the bud but it's still consistently coming out at different times."
Postecoglou also said: "Look, I've always said, especially with medical information, I'm really, really careful.
"You try to keep the circle of information pretty tight. I don't understand why people would do it, especially if they're so-called in our camp because I don't know how it's helpful to us.
"Even though we're in the world of football and I know everyone thinks these things should be carte blanche in terms of information flow, I'm still very respectful in terms of medical records and how much information we actually give on certain things.
"Sometimes players don't want that information disclosed, it's as simple as that. The fact that it is getting out is unacceptable.
"As a club, I just think, I've got enough challenges out there without adding more to ours. We'll deal with it internally. Like you deal with anything. You do the right thing and deal with it."
Postecoglou says shutting down the mole is part of the change in culture he is trying to bring through the club.
"You'd like to think that everyone within our camp is working with us rather than against us," Postecoglou said.
"I think it's a cultural thing. I'm big on creating a culture of success and I don't think successful organisations behave in that manner.
"That's the bit that we're constantly, not just me, but the other people in the club are trying to change and trying to get the club to a place where there is a real strong mentality and identity of who we are and protect that.
"If you have people giving out sensitive information, particularly on the medical front or the tactical front, then you need to address it."