The Dutch romped to a 5-1 win over Spain in their opening fixture, while Australia are still looking for their first World Cup points after losing to Chile 3-1 last week.
The Australian manager is all-too aware of the Dutch threat going forward, but knows his side cannot sit back and defend for the entire game.
"We know we have to be strong defensively, the Dutch are dangerous going forward as they showed against Spain," said Postecoglou.
"We know that van Persie and Robben will be a challenge but we knew that coming into the tournament.
“In saying that we also need to be attack-minded, we can't just defend for 90 minutes."
Postecoglou candidly admitted that a negative result will spell the end for Australia’s campaign at this year’s World Cup.
"We just have to believe in what we do and [on Wednesday] we will get a chance to find out where we are at," he said.
"There's a lot at stake for both teams. For us if we don't get a result it's the end of our tournament."
Meanwhile, opposite number Louis van Gaal is not letting his side get carried away after their opening victory, and is far from under-estimating Australia ahead of their match-up in Porto Alegre.
"You win the World Cup by winning the final," said the Dutch coach.
"This is just the first step. We have to keep our feet on the ground, we shouldn't get ahead of ourselves.
“We need to prove our capability to win over Australia, which won't be that easy.”