"Any team can be beaten on their day," he says.
"And that is obviously our intent."
Postecoglou and his Socceroos are travelling to Brazil and as rank underdogs.
Australia is ranked 59th in the world - the lowest of any competing nation in football’s showpiece.
Some pundits believe Australia will be lucky to score a goal, let alone scrape a point against Chile, Netherlands and reigning champion Spain.
But that doesn't bother the unflappable Postecoglou, who has embarked on a regeneration of the Australian side.
He's planted the seeds. But he's uncertain just when they will flower.
Postecoglou is adamant Australia can't keep relying on Tim Cahill, aged 34, new captain Mile Jedinak, 29, and midfielder Mark Bresciano, 34.
Instead, he's looking to establish the talent of players such as Tommy Oar, Matthew Leckie, James Holland and Jason Davidson - with just 40 internationals between them - to usher in generation next.
"We need to get games into these guys," Postecoglou says.
"They have got to get up to 15, 20, 25 caps.
"And you can see they're starting to grow into it."
The Australians fly out with lingering injury clouds hovering over several players, including Jedinak.
The new skipper is unsure when he'll play - hoping it's in the warm-up game against Croatia on 6 June, seven days before their World Cup opener.
Jedinak realises he's taken charge of a callow squad - only Cahill, himself, Bresciano, Luke Wilkshire, Josh Kennedy and Matt McKay have played more than 30 internationals.
But Jedinak says his personal battle with a groin injury has given him a greater appreciation of the talent within the Socceroos.
"I do rate them very highly," Jedinak says of his team-mates.
"One thing with having this time on the sidelines, it has allowed me to take notice of people on the field and be able to watch training.
"And the boys are working really hard ... you can see that it means an awful lot to everybody."
Jedinak notes improvement in Postecoglou's three games in charge - a 1-0 win against Costa Rica last November, a 3-4 loss to Ecuador in March and a 1-1 draw with South Africa in Sydney on Monday night.
He says players are becoming comfortable with Postecoglou's demands for a high-tempo, possession-based style.
"We have tried to implement a style which the boss is encouraging," Jedinak says.
"You have seen a progression of that all the time.
"We are being challenged at training to do that. And the reward is taking it out and doing it on the pitch in the games.
"We have been working towards it ... day by day, that is improving. And we're demanding top, top quality."
For Postecoglou, the 1-1 scrap with South Africa was far from a disaster.
After a sparkling initial 25 minutes, the Socceroos failed to make a dent on the 66th-ranked opponent but the coach puts it down to fatigue, given their heavy training workload.
"I know where we're at," Postecoglou says.
"You want to win every game, it would be great. Everyone would have a warm and fuzzy feeling if we won.
"We tried a few things that we need to try now because there is no point trying them when the World Cup is on.
"From our perspective, it's about being ready for game one in the World Cup."
That game is against Chile on 13 June, with fixtures against Netherlands (18 June) and Spain (23 June 23) coming afterwards.
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