A report on the amended bill was adopted by unanimous vote by the ruling party and will now be debated in the National Council of Provinces of Thursday, in one of the last steps before it becomes law.

Opposition parties had welcomed eleventh-hour changes brought by the ANC, but refused to be party to the adoption of the report because they did not have time to study it.

"We were told we would have the report at the weekend, not half-an-hour after the meeting began," Democratic Alliance MP Alf Lees said.

He said it was not proper to read the report in a few minutes to see whether it accurately reflected both amendments agreed to and dissenting minority reviews.

Committee chairman Raseriti Tau lamented the walk-out and pre-emptively rubbished remaining criticism of the bill as grand-standing, given the number of amendments made to it, which he put at 800.

"Surely, after such an elaborate consultation process and thorough amendment to address the challenges and concerns raised about the Info Bill, one would expect general support of the bill," he told a media briefing.

But the Right2Know campaign, which was born in opposition to the bill, said the ruling party had only partially addressed concerns; loopholes remained, and the result was likely to be over-classification.

The ANC's latest changes to the bill were brought last week after the party came under renewed pressure from its ally Cosatu.

It notably went against the wishes of State Security Minister Siyabonga Cwele by proposing to reinsert a clause offering protection for those who reveal classified information to expose a crime.

It also agreed to scrap a clause that would have made the new legislation trump the progressive Promotion of Access to Information Act, which gives citizens the right to access the information necessary to exercise their constitutional rights.

Thirdly, it moved to allow the Public Protector, the Auditor General and all other chapter nine institutions to be in possession of classified information.

Madonsela met with the ANC again last week after warning in March that she could be arrested for routinely receiving leaked documents from whistle-blowers.

The bill has undergone two-and-a-half years of redrafting since it was introduced in 2010, and was slated by some as a throwback to apartheid era state secrecy.

The sustained outcry over the bill saw the ANC make a phased retreat from its most contested provisions, including those imposing minimum prison sentences for releasing secret documents and enabling, literally, 1001 state entities to classify information.

On Tuesday, Tau blamed the controversy on misrepresentation by opponents of the bill, and went on to make the surprising claim that the draft law included a public interest defence.

Such a defence, which would allow reporters and whistle-blowers to argue in court that they had revealed state secrets for the greater public good, has been a rallying cry of civil rights groups and Cosatu.

Cwele memorably said the ANC would never insert such a defence as it would be equal to tearing up the bill.

Tau claimed it was contained in clause 19, which obliges the intelligence minister to grant a request to declassify information if the public interest outweighs the security risks in releasing it.

The Right2Know Campaign said the bill, after last week's changes, finally included "a limited public interest exemption" in that whistle-blowers were protected if they published classified information to expose a crime.

But it did not go far enough, and failed to protect those who sought to alert the public to an imminent health or environmental threat.

Furthermore, Right2Know said, the bill still criminalised mere possession of secret documents, as well as criminalising the publishing of classified information once it was already in the public domain -- as the media had done with Wikileaks exposes.

"This is so blunt an intrusion on the rights to access to information and freedom of expression that it will fail the constitutionality test," it said, reviving the threat of a legal challenge to the bill.

The bill will be debated in the NCOP on Thursday.

After its likely adoption by the council, given the ANC's majority in the chamber, it has to be referred back to the National Assembly for approval after Parliament reopens next year.