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GCIS acting CEO Phumla Williams is expected to shed light on former president Jacob Zuma's alleged involvement in diverting state funds to the Gupta family.
Williams will tell the Zondo commission, under oath, what her role was in the payment of millions in state funds to Gupta-owned companies.
A source with knowledge of the evidence said her affidavit goes through a chronology of events from the time she was appointed acting head of the Government Communication and Information System, until now.
Williams has held the position for six years.
Contacted for comment, Williams said: "I will tell my full story when I testify ..."
Though Williams claims to have been acting under political instructions, her role in assisting the Guptas to profit irregularly from government advertising is being probed.
Former GCIS head Themba Maseko on Thursday detailed how the Guptas aimed to divert all government advertising expenditure to themselves. He is expected to finish his testimony today.