Prime Minister Tony Abbott personally approved a decision to make cabinet documents available to the royal commission into the Rudd government's botched home insulation scheme.
Former prime ministers Bob Hawke and Malcolm Fraser expressed alarm at the decision to hand over cabinet documents, revealed by Fairfax Media on Saturday, because they fear it will endanger the convention of cabinet confidentiality.
Deputy secretary of the Department of Prime Minister and Cabinet, Elizabeth Kelly, told Senate estimates hearings on Monday that 4500 government documents, including cabinet documents, have been handed to the royal commission since January.
The department provided cabinet documents to it as recently as February 11, Ms Kelly said.
The decision to comply with a summons for documents from the royal commission was approved by Mr Abbott, she said, and neither the Prime Minister nor anyone from his office had seen the documents, which were passed to the Attorney-General's Department to send to the commission.
The cabinet documents were given to the royal commission on the condition they be viewed privately. If the commission wants to make any of the documents public, the Department of Prime Minister and Cabinet has requested it be told in advance so it can apply for a ''public interest immunity'' waiver to block publication.
Ms Kelly said cabinet documents had previously been provided to two royal commissions (the Centenary House investigation and the inquiry into the Mohammed Haneef affair).
Ms Kelly's explanation came only hours after Attorney-General George Brandis and his department secretary, Roger Wilkins, told a separate Senate estimates committee they were not aware of any requests from the royal commission for cabinet documents.
Mr Wilkins later issued a clarification, saying he did not ''want to mislead the Senate'', saying ''the [Attorney-Generals'] department will act as an agent … as a go-between in terms of the various departments and the royal commission in terms of the answering of summonses and subpoenas''.
Labor Senator John Faulkner claimed the case was an ''open-and-shut'' breach of 113 years of cabinet confidentiality.
According to the Australian Government Cabinet Handbook: ''The convention is that cabinet documents are confidential to the government which created them.''
Government Senate leader Eric Abetz said it would have been absurd for the Prime Minister's department not to comply with a summons from the royal commission given this constitutes a criminal offence.