Transnet has appointed company insider Mohammed Mahomedy as its second acting chief executive in less than seven months as it continues to investigate graft allegations.

Transnet, which operates nearly three-quarters of Africa's rail network, the bulk of which is in South Africa, is looking into allegations of corruption in the procurement of diesel and electric locomotives.

The board said in a statement late Wednesday it had decided not to extend the contract of acting group chief executive Tau Morwe, whose contract ends on Friday.

It did not give a reason, but thanked Morwe for his good work. Morwe could not immediately be reached for comment.

The group said the appointment of Mahomedy, who has more than 12 years of experience at Transnet and was most recently acting group finance chief, would help to ensure stability at the organisation while it looks to fill top positions.

"Transnet is working towards filling critical vacancies that currently exist at executive level, including that of the Group Chief Executive and Group Chief Financial Officer, in the coming months," it said.

The board removed former Chief Executive Siyabonga Gama last year after he was accused of misconduct in a multi-million-dollar deal. Gama has denied the accusations.

Transnet is one of a handful of South African state firms that became embroiled in corruption scandals under former President Jacob Zuma.

New President Cyril Ramaphosa has launched an inquiry into allegations that tenders were awarded illegally during his predecessor's time in office. Zuma has denied any wrongdoing and says he is the victim of a politically motivated witch-hunt.