The intercepted phone conversations that torpedoed the case against ANC president Jacob Zuma should never have been made public, according to former National Assembly speaker Frene Ginwala.

The SABC reported on Thursday that Ginwala said this was her understanding of the law.

She was speaking on the sidelines of a memorial service in Pretoria for late communications minister Ivy Matsepe-Cassaburri.

Zuma's lawyer, Michael Hulley, has refused to say how he got the tapes, that prompted the National Prosecuting Authority (NPA) to withdraw its corruption and fraud case against Zuma.

The National Intelligence Agency (NIA), which was called on by the NPA to verify transcripts of the tapes, has strongly denied that they were leaked to the Zuma camp by deputy intelligence chief Arthur Fraser.

Acting NPA boss Mokotedi Mpshe said on Monday he could not proceed with the case because the tapes showed that former Scorpions head Leonard McCarthy had committed a gross breach of procedure by discussing the timing of charges with former NPA chief Bulelani Ngcuka.

The Office of the Inspector General of Intelligence has confirmed that it is probing how the tapes got to Zuma, as well as whether the NIA was acting within the law when it intercepted the calls.