"It's deeply problematic."
In June, the ANC buckled to pressure to limit the scope of the law to protect information that had the potential to undermine national security, rather than the broader national interest.
It rejected calls for the insertion of a clause allowing such information to be released in the public interest.
The Freedom Front Plus, an opposition party, said it would ask the Constitutional Court, the country's highest judicial body, to determine whether the omission of a public interest defence was lawful.
"We believe that the exclusion of the defence gives too much power to the Government to classify information as confidential and will therefore not withstand the test of constitutionality," the party said in an emailed statement.
The bill proposes that anyone who releases or receives top secret information for the purposes of espionage be jailed for between 15 years-25 years.
It envisions sentences of between three-15 years for lesser offences relating to the access and possession of classified documents.
"The penalties are incredibly harsh" and will serve as a deterrent to whistle-blowers, Right2Know's Hunter said in an interview in Cape Town.
Right2Know represents more than 400 organisations and 16,000 individuals.
Its backers include cleric Desmond Tutu and author Nadine Gordimer, both of whom are Nobel laureates.
- BLOOMBERG