Attorney-General George Brandis has cited Cabinet confidentiality as being behind his decision to reject a Senate motion for the government to release a PricewaterhouseCoopers report on the cost of the government's data-retention legislation.

Legislation currently being reviewed by the Joint Standing Committee on Intelligence and Security would require Australian telecommunications companies to retain a set of customer information, including IP addresses, call records, and other personal information for a period of two years for warrant-less access by designated law-enforcement agencies.

The legislation has been resisted by a number of telcos on cost grounds, as well as civil rights and privacy advocates, due to the associated privacy implications with a large wealth of data collected over that two-year period.

Communications Minister Malcolm Turnbull announced in introducing the legislation in October that the government would "make a substantial contribution" towards the cost of setting up and running the scheme for the more than 200 telecommunications providers in Australia, but the actual overall cost of the scheme remains a mystery, even to the government.

As noted in a Parliamentary Library paper last week (PDF), the cost of mandatory data retention in Australia is unknown, ranging from AU$100 million by the Australian Mobile Telecommunications Association in 2012, up to AU$400 million according to iiNet. Estimates on the cost for the UK's own data-retention scheme have been placed at £1.8 billion over 10 years.

The government commissioned PricewaterhouseCoopers to determine the cost of the proposal in October; however, the report itself has remained confidential. In November, Greens communications spokesperson Scott Ludlam successfully passed a motion in the Senate that the government tabled in the report.

"It amounts to little more than a surveillance tax -- one that was not announced before the election. Industry presumably will pass on some of these costs to users, and the rest will be picked up by taxpayers," Ludlam said at the time.

"The PwC report has been commissioned to find out how much it will cost. I think it is imperative that the government puts that report into the public domain so that we know what the cost will be."

Brandis declined to table the report, and in a second successful motion, in early December, Ludlam sought a summary of the report, or the report with the commercially sensitive parts removed.

In response, on Thursday last week, Brandis again rejected the motion.

"The relevant report was commissioned and prepared for the dominant purpose of briefing the prime minister, the attorney-general, the minister for communications, and Cabinet," he said.

"Disclosure of the information sought ... in relation to the findings of this report would reveal a Cabinet deliberation. Therefore, I once again respectfully advise that I am not in a position to comply with this order."

Brandis said that the government has set up an "Implementation Working Group" for the data-retention legislation as part of its engagement with the telecommunications industry, and the PwC report was an "intermediate step in the costings exercise" of the scheme.

PwC is also continuing to support the group and the Attorney-General's Department in the development of the data-retention scheme.

It follows a decision by Brandis' chief of staff, Paul O'Sullivan, to also censor the attorney-general's own metadata sought under Freedom of Information. The Telstra bill, paid for by Australian taxpayers, was censored almost completely due to privacy concerns. The decision is now being considered for review by the Office of the Australian Information Commissioner.