The Office of the Inspector General of Intelligence (OIGI) is probing whether the intelligence services were acting within the law when they made the spy tapes that sank the State's case against African National Congress (ANC) leader Jacob Zuma.

"The circumstances surrounding the interception of the voice communications of certain individuals by the Intelligence Services is currently being investigated," OIGI's COO Imtiaz Fazel said on Wednesday.

"The investigation is seeking to establish whether or not these communications were lawfully obtained by the services as required by interception legislation and handled in a manner that is compliant with appropriate legislation and the constitution."

Fazel said the scope of the investigation was confined to the conduct of the intelligence services and declined to provide further details.

On Monday, acting National Prosecuting Authority (NPA) head Mokotedi Mpshe, cited extracts from transcripts of phone recordings as proof that the then Scorpions boss Leonard McCarthy took political instructions on the timing of recharging Zuma in 2007.

Also quoted was former NPA chief Bulelani Ngcuka.

Though no longer in the job, he appeared in Mpshe's presentation to be dictating to McCarthy when to proceed with the charges, given the sensitivity of the Polokwane conference where Zuma and the then President Thabo Mbeki battled for control of the ANC.

Mpshe said that after Zuma's lawyers confronted his staff with recordings to this effect, he approached the National Intelligence Agency (NIA) to verify the authenticity of the tapes.

The NIA produced its own tapes that matched those in Zuma's possession, he said as he announced he was dropping all charges against the ANC's Presidential candidate.

Mpshe said the NIA recordings were made legally as part of an investigation into the controversial Browse Mole report produced by the Scorpions that outlined an alleged foreign-backed plot to overthrow Mbeki.

The Sunday Times reported last week that permission to tap McCarthy's phone was granted by a High Court judge after the police presented an affidavit alleging he was involved in crime.

But the newspaper said it found no evidence of a criminal investigation into McCarthy and could not establish the name of the judge who granted the order.

The OIGI has confirmed that it was also probing how Zuma's lawyer got hold of the secret tapes.

The Argus on Wednesday reported that McCarthy had signalled that he would cooperate with a judicial commission of inquiry into the Scorpions' handling of the Zuma case, provided he was furnished with the recordings that were cited by Mpshe.

Mpshe concluded that it was not possible to proceed with the corruption and fraud case against Zuma because McCarthy's conduct amounted to "a serious abuse of process".