The Department of Prime Minister and Cabinet (PM&C) has ordered an urgent investigation into the release of hundreds of top-secret and highly classified cabinet documents.

Key points: Hundreds of highly classified documents have been revealed after they were found in filing cabinets at a second-hand shop

Hundreds of highly classified documents have been revealed after they were found in filing cabinets at a second-hand shop Documents reveal national security breaches, NBN negotiations, welfare reform plans

Documents reveal national security breaches, NBN negotiations, welfare reform plans PM&C has not specified when the investigation into the security breach will begin or who will conduct it

PM&C secretary Martin Parkinson called for the inquiry less than an hour after the ABC revealed one of the biggest breaches of cabinet security in Australian history.

Some of the cabinet documents are classified as "top secret" or "AUSTEO", which means they should be seen by Australian eyes only.

They should have remained secret for 20 years but have become public because they were left in two locked filing cabinets that were sold at an ex-government furniture sale in Canberra.

The documents revealed information about Kevin Rudd being warned about "critical risks" of the home insulation scheme, Scott Morrison's decisions as immigration minister, and the approach Tony Abbott's "razor gang" took to welfare for young people ahead of the 2014 budget.

Mr Parkinson said now the urgent investigation had commenced it would be inappropriate to comment further.

He did not say how long the investigation would take or who would be conducting it.

Former prime minister Tony Abbott said Dr Parkinson should explain because the documents were "apparently in the custody of the department".

"The department has a responsibility to keep them safe," Mr Abbott told 2GB radio.

"Obviously some absolutely elementary mistake has been made, presumably by a relatively junior or mid-ranking departmental officer.

"Certainly someone needs to pay a price, there needs to be some consequence for what is a monumental lapse."

Mr Abbott said the filing cabinets containing the highly classified documents were not thrown out from Parliament House.

"It was thrown out — so to speak — by the Department of Prime Minister and Cabinet," he said.

"So just for once the politicians aren't to blame here.

"This is something that is purely and simply the responsibility of officialdom and they need to give an account of themselves."

He went on to state the problem in simple terms.

"If you are going to throw out a filing cabinet that has got a whole lot of sensitive and confidential information in it you have got to make sure it is empty before it goes out of the building."

The extraordinary breach is expected to be raised next month when his department appears before a Senate estimates committee.

'Sloppy safeguarding' blamed for breach

Rory Medcalf, head of the national security college at the Australian National University, described the breach as "an affront to national security personnel".

"National security personnel work hard and diligently to prepare classified briefing material to serve the national interest," he tweeted.

"It is an affront to their general professionalism that someone, somewhere was so sloppy with safeguarding it."

Penny Wong was among the politicians linked to documents contained in The Cabinet Files.

She told the ABC she had not known that nearly 200 top-secret code word protected and sensitive documents were left in her office when Labor lost the 2013 election, until The Cabinet Files were published by the ABC.

"This is the first time I have ever been made aware of this matter, which relates to a change of government over four years ago," she said in a statement.

"As a former cabinet minister who participated in national security meetings, a senior member of shadow cabinet and a current member of the Parliamentary Joint Committee on Intelligence and Security, I always take my responsibilities seriously."

Liberal backbencher Craig Kelly criticised the ABC's decision to publish the details of the breach, saying it had not acted in the national interest.

"There's taxpayers' money funding the ABC, $1 billion goes into funding ABC," Mr Kelly told radio 2GB.

"If they get hold of the things marked confidential, government secrets, I think there's an obligation upon the ABC to contact the AFP and say 'look we've come across these documents' and hand them back to the AFP."