Public Enterprises Minister Lynne Brown announced late on Monday night that Ngubane had resigned from his post. Chief whip on the ANC Study Group on public enterprises,
Zukiswa Rantho, said: "As chairperson of the board, Dr Ngubane was central in processes which led to the now rescinded reappointment of Mr Brian Molefe at Eskom.
"Furthermore, Parliament's portfolio committee on public enterprises had presented a number of questions to the Eskom board which they are yet to come back to parliament to respond to. The timing and rationale of this resignation is therefore surprising and suspect," Rantho added.
"It cannot be correct that board members chose to resign when they are to be held accountable for their actions and decisions in our state-owned enterprises. This sudden resignation must therefore be followed by an investigation into allegations of misconduct and corruption at Eskom."
Earlier, the opposition Democratic Alliance (DA) said that the resignation of Ngubane would not exonerate him from liability for the breakdown of governance at the power utility.
DA spokesperson on public enterprises Natasha Mazzone said in a midnight statement that Ngubane still had much to answer for as his resignation came "hot on the heels of damning revelations of the capture of Eskom leadership by the Guptas, and the breakdown of corporate governance at Eskom".
"Mr Ngubane, who presided over the retrenchment/retirement/rehire of Brian Molefe as Eskom CEO has much to answer for, not least of which includes Molefe's R30 million 'golden handshake' deal and an avalanche of accusations that the Guptas have milked Eskom over many years.
"Ngubane's resignation will not exonerate him from liability for the breakdown of governance at the power utility, and the DA will use the Parliamentary Inquiry into Eskom, set to commence on 21 June, to hold Ngubane to account," Mazzone said.
Mazzone said that allegations by former Mineral Resources Minister, Ngoako Ramatlhodi, that Molefe and Ngubane had tried to strong-arm him to withdraw Glencore's Optimum mining licences in order to guarantee that the Guptas takeover Glencore’s coal mines had yet to be probed.
Brown appointed Zethembe Khoza as the interim board chairman "until I am able to take new board appointments to Cabinet for approval".
She had recently said that she planned to "rotate" the board.