The tourists went into the series as firm favourites after their 3-0 victory in the series in England earlier this year. (Agencies)
But even Cook conceded they were completely outplayed in the first two Tests in Brisbane and Adelaide, and again in Perth - where they lost by 150 runs to give the home side an unbeatable 3-0 lead with two Tests left to play.
A feature of the series has been the poor performance of senior English players, including Cook himself, powerful batsman Kevin Pietersen, wicketkeeper-batsman Matt Prior, spinner Graeme Swann and swing bowler Jimmy Anderson.
Cook, who suffered his first series defeat as skipper, told reporters he realised there would be a detailed autopsy of the team's failure.
He accepted that key England players were down on their best form in the first three Tests, but said it was premature to read anything more long-term into it.
"I wouldn't necessarily say that," he said when asked whether the loss signalled an English team in decline. What I would say is, unfortunately, when we needed people to be in form and playing well, we haven't done that, and that's why we lost.
"We haven't done ourselves justice out there in these three games, people haven't performed like we know they could have done, and it's frustrating when that happens."
Cook pointed to the Australians, with veterans like Brad Haddin and Mitchell Johnson playing some of the best cricket of their careers, as evidence that form can be turned around by experienced players.
The tourists went into the series as firm favourites after their 3-0 victory in the series in England earlier this year.