Vanara asked Ngubane to be plain about the content of the phone call and whether Brown made sure Koko was given a reprieve.
"What did the minister call to talk to you about? Don't go around in circles."
Ngubane replied: "This was a normal call."
Vanara shot back: "I give up Doc."
Last year, Khulani Qoma, the now suspended spokesman for the Eskom board, told the inquiry he had urged chairman Zethembe Khoza to suspend Koko because of the reputational damage he believed he was inflicting on the company.
Qoma said Khoza fairly readily agreed that it should be done, but cautioned that Ngubane had also wanted to move against Koko but was blocked by the minister.
“He then tells me that actually Dr Ngubane came very close to suspending Koko, but was stopped in his tracks by Minister Brown," he testified at the time.
He said Ngubane had persuaded the rest of the board that this was the correct course of action, but the Gupta brothers -- the family at the centre of allegations of wholesale corruption at Eskom and other state-owned enterprises -- were called to inform them of the decision and they called Brown, who in turn called Ngubane and ordered him not to do it.
Vanara on Wednesday reminded Ngubane of Qoma's testimony, saying: "There is evidence before this committee that you received a call from the minister where the minister instructed you not to suspend Mr Koko."
But Ngubane insisted: "I don't know where that comes from."
He claimed instead that the decision not to suspend Koko was based on the fact that the board had received incriminating information from a whistleblower and that it was illegal to act on that directly without going through a prescribed process.
“He would have taken us to the CCMA and labour court and won," he said.
Ngubane resigned as Eskom chairman in July last year. Brown was fired from Cabinet by President Cyril Ramaphosa last week. Koko resigned from Eskom last month.