Kevin Rudd said the release raised questions about US security.
Mr Rudd said he did not “give a damn” about criticism of him in the cables.
Mr Assange, arrested in the UK over sex crime allegations in Sweden, has accused the Australian government of “disgraceful pandering” to the US.
Prime Minister Julia Gillard had called the release of the cables “grossly irresponsible”.
Over the past two weeks, Wikileaks has released thousands of classified messages from US envoys around the world, from more than 250,000 it has been given.
Washington has called their publication “irresponsible” and an “attack on the international community”.
‘First class job’
In an interview with Reuters news agency, Mr Rudd said: “Mr Assange is not himself responsible for the unauthorised release of 250,000 documents from the US diplomatic communications network. The Americans are responsible for that.”
Mr Rudd, the former prime minister who was replaced by Julia Gillard in June, added: “I think there are real questions to be asked about the adequacy of [the US] security systems and the level of access that people have had to that material.
“The core responsibility, and therefore legal liability, goes to those individuals responsible for that initial unauthorised release.”
Mr Rudd was dismissed in one leaked US cable as a “mistake-prone control freak”.
In cables published by the Sydney Morning Herald, former US ambassador Robert McCallum said Mr Rudd made “snap announcements without consulting other countries or within the Australian government”.
Mr Rudd shrugged off the criticism, saying: “I’m sure much worse has been written about me in the past and probably much worse will be written about me in the future but frankly, mate, I don’t care.
“My job’s just to act in Australia’s national interest as Australia’s foreign minister. I don’t, frankly, give a damn about this sort of thing. You just get on with it.”
Ms Gillard defended Mr Rudd, saying: “He’s bringing [his] expertise to bear for the Australian nation and doing an absolutely first class job.”
Mr Assange has been highly critical of the Australian government’s stance on the release of the cables.
In an opinion piece in the Australian on Wednesday, Mr Assange accused the Australian government of “disgraceful pandering” to the Americans and of putting the powers of the government fully at the disposal of the US.
In the piece headlined “Don’t shoot the messenger for revealing uncomfortable truths”, he says: “Democratic societies need a strong media and Wikileaks is part of that media. The media helps keep government honest.”