Having been hammered 4-0 by Brazil in Melbourne last week in a friendly ahead of the tournament in Russia, Australia stumbled again.
Having been hammered 4-0 by Brazil in Melbourne last week in a friendly ahead of the tournament in Russia, Australia stumbled again, losing to the world champions, who fielded an inexperienced side on Monday in Sochi.
Lars Stindl, Leon Goretzka and Julian Draxler -- the latter from a penalty -- scored for Germany, with Celtic midfielder Tommy Rogic and Tomi Juric netting for Australia.
Juric's effort on 56 minutes was allowed to stand despite a suggestion of handball leading to a review from the video assistant referee.
Having found themselves 3-1 down just after the break, Australia rallied in Sochi and Juric's effort reduced the deficit before he, Robbie Kruse and James Troisi all tested Germany goalkeeper Bernd Leno late on.
There was to be no equaliser, and Postecoglou was especially disappointed about the way his side started the game.
"It's disappointing, we were barely clinging on in the first half," admitted Postecoglou.
"The second half was better, we composed ourselves and got back into things."
"It's a loss and the loss falls on me, it's my responsibility."
"It was no fault of the players, they (Germany) took advantage in the first half and they were way too good."
The Socceroos, who lost all three games on their last appearance at the Confederations Cup in 2005, now face a crunch clash with the African champions in Saint Petersburg in their second Group B outing.
Cameroon are unlikely to offer the same threat as the Germans, even if the world champions have come to Russia without a host of leading players.
"There is no question, it's a team full of character and it's not easy to play well against the best team in the world," said the 51-year-old Postecoglou.
"They (Germany) caused us problems down the left-hand side and we had to move things around after the break."
"They probed for weaknesses, they capitalised on the opportunities they had."
"We sat back too deep. As much as you try and set up the team to be brave in terms of where we want to defend, it's human nature to pull back and sit deeper than you'd like."
However, his opposite number Joachim Loew said Australia have developed since Postecoglou took charge in 2013.
Indeed, Loew believes the Socceroos have come a long way since their 4-0 rout at the hands of Germany in the group stage of the 2010 World Cup.
"From what I have seen there has been a positive development from where Australia was in 2010 -- they liked long balls then and that was easy for us to defend against," said Loew.
"They aren't playing long balls any more, they are courageous and cheeky enough to boldly attack."
"Not many teams are doing that and they have developed positively in the last three years."
Loew has said there is much more to come from his young squad and his starting line-up in Sochi had only 138 international appearances between them.
With all of his World Cup-winning stars left at home, Loew warned Chile on Thursday in Kazan will be a tough test for a youthful side.
"Who'd expect everything to run top notch after only 10 days together? We have done really well for the time we had together," said Loew.
"Not everything went well and we have to correct things."
"Chile will be a different game, they play with a lot of intensity and we will need to play well over 90 minutes, not just over 60."
"Chile is one of the world's best teams, not just in this tournament, they have a lot of experience and fantastic individuals."