South Africa's anti-corruption watchdog said on Thursday that it was investigating Public Enterprises Minister Pravin Gordhan, a close ally of President Cyril Ramaphosa, for allegedly abusing his powers during his time as finance minister.

The watchdog said it was looking into allegations that Gordhan wrongly approved an early retirement package for a senior tax official at the South African Revenue Service (Sars) in 2010, before allowing him to be re-employed at the agency on a contract basis.

Gordhan is currently leading efforts to shore up the country's debt-ridden state-owned companies, which have been hobbled by years of mismanagement.

Public Enterprises Ministry spokesperson Adrian Lackay said on Thursday that Gordhan was not able to comment because he had made a submission on the investigation to a separate judicial inquiry into state corruption headed by Deputy Chief Justice Raymond Zondo and was not allowed to disclose that submission.

Gordhan has said in the past that the early retirement of tax official Ivan Pillay was entirely above board.

The Public Protector, an organisation mandated by the constitution to uphold standards in public office, said the investigation into Gordhan had started in 2016 but had now reached a stage where it was ready to conduct interviews.

"The Public Protector subpoenaed and is investigating allegations of impropriety against Minister Gordhan," said Oupa Segalwe, a spokesperson for the Public Protector.

"It has been alleged that the minister approved Pillay's retirement and bought off his pension balance irregularly and later allowed him to be re-employed by the South African Revenue Service," Segalwe added.

In an opinion piece in a South African newspaper last month, Gordhan said his efforts to clean up corruption in state-owned companies faced "dangerous" resistance which threatened the country's sovereignty.

Gordhan has some powerful enemies in the ruling African National Congress and has been criticised by the radical Economic Freedom Fighters party as the power behind the throne.

Ramaphosa has said Gordhan needs to be applauded for the actions he has taken as public enterprises minister.

Judge Zondo is leading an inquiry into claims of influence-peddling against former president Jacob Zuma.

The inquiry will review allegations that the three Gupta brothers unduly influenced Zuma over political appointments and winning government contracts. Both Zuma and the Gupta brothers deny wrongdoing.