Four weeks out from the official announcement, Postecoglou said while there is still time for fringe Socceroos players to make an impression in their domestic leagues, the core of his squad was firmly set in his mind.
Australia blitzed Ecuador in the opening 45 minutes of a friendly earlier this month to lead 3-0 at half-time but were destroyed after the break to lose 4-3.
Despite that result, Postecoglou hinted the majority of players used against Ecuador would be going to the World Cup.
"I do (have a squad in mind), and I guess there's always things that are coming across on a weekly basis," Postecoglou said on ABC program Offsiders.
"There's always someone who'll score a goal who should be on the plane from every weekend's game of footy.
"From my perspective, and particularly after the Ecuador game, there's a real clear understanding from my behalf in terms of the direction we need to go.
"And then you're fitting in the pieces of the puzzle to do that.
"There's obviously going to be some variance in the final few spots, but the core of that team, and the philosophy I want to take moving forward which I believe we need to take, is absolutely crystal clear now."
Postecoglou said he will personally break the news to those players who he leaves out of the squad to Brazil, with several veterans including Lucas Neill, Brett Holman and Sasa Ognenovski potentially missing the cut.
"With quite a few of them having distinguished service for your national team... it does require me to give notice before the squad's announced I think," he said.
"When you work back from that squad, that time is fast approaching."
