The South Gauteng high court has ruled that the government's use of the Regulation of Interception of Communications and Provision of Communication-Related Information Act - commonly known as Rica - to place journalists under surveillance is unlawful.

The ruling comes after investigative journalism centre amaBhungane filed a challenge to the court in 2017 to review the use of Rica to place their managing partner, Sam Sole, under surveillance.

Sole's phone communications were intercepted when he led an investigation into the NPA's decision to drop corruption charges against former president Jacob Zuma - prompting amaBhungane to demand a reason behind the decision.

It was then confirmed that a judge's order had been signed to allow the use of Rica, although the intentions behind the surveillance were not disclosed.

Non-profit organisations Right2Know and Privacy International argued earlier this year in the Pretoria high court that bulk surveillance was unconstitutional, before joining amaBhungane in its challenge.

Also earlier this year, amaBhungane said that while a positive judgment would have serious implications for any state body trying to gain access to private citizens, it was by no means a "silver bullet" as it was limited to the facts relevant to Sole's specific case.

Judge Roland Sutherland ruled on Monday that mass surveillance and the interception of foreign signals by the National Communications Centre (NCC) was "unlawful and invalid" in terms of the constitution.

Sutherland said that in order to ensure that whistleblowers were confident that they could disclose information about their bosses while having their identities protected, the use of "bulk interceptions" could not be used.

"The declaration of invalidity is suspended for two years to allow parliament to cure the defect," he added.