An investigation into all private companies that do business with the South African government is long overdue, said African National Congress (ANC) secretary general Ace Magashule.

Speaking to reporters on the sidelines of the state capture commission of inquiry on Tuesday, Magashule said the controversial Gupta family has been targeted while other companies are never probed. 

"I do not know why South Africa is not actually investigating every company which has done work with government. Only one particular family company is targeted. Go to government and find out who has done business with government...I was a provincial leader. Big companies make billions while young black people's companies get 20 projects worth R5-million. There is no commission that has ever pronounced on that. South Africans are not stupid, we can't be quiet forever, we must speak out."

He added that the state capture commission is the right platform for everyone to speak out. 

The commission, led by Deputy Chief Justice of South Africa Raymond Zondo, was established to probe the alleged looting and hollowing out of the State by political figures, the fugitive Gupta family and others.

Asked what he makes of alleged death threats against Zuma, Magashule said other leaders had also received threats previously.

"I do not want to enter that space...it is what he is saying. You do know what [former president Nelson Mandela] said in the past around this matter, I am sure you also know what Oliver Tambo said too in the past. When you are a leader of an organisation, you are that organisation and you keep the resolutions...not the fashion we see now where we discuss matters in the national executive committee and those get leaked. You cannot have a leadership running the country and leaking information as if the media runs the ANC."

On his threat to take legal action against journalist Pieter-Louis Myburgh who wrote the book Gangster State in which he alleged Magashule to be central to a network of patronage and corruption, the ANC leader said his lawyers were on it.

"My lawyers are dealing with that matter, you guys want to push me and then wrong things happen. But I will never leave it...I will not leave that man," he said.

Magashule joined the public gallery at the inquiry on Zuma's second day on the stand.