Fraudster Schabir Shaik's case was one of "parole for pals", Democratic Alliance leader Helen Zille said on Thursday after her party filed an application demanding insight into the medical grounds for his release.

"In the ANC's closed, crony society, people get preferential treatment if they have friends in high places," Zille told students at the University of Cape Town.

"That explains why Jacob Zuma's former financial advisor Schabir Shaik can be fast-tracked for release on medical parole - without official confirmation that he is terminally ill -- while hundreds of other terminally ill prisoners die waiting for their parole applications to be processed.

"Shaik's discharge is a case of 'parole for pals'."

Zille said Shaik's release from prison on Tuesday was a step on the way to being pardoned by Zuma if the ANC leader were to become president after the April 22 elections.

"Shaik's parole is just a first step. In a recent interview, Zuma hinted strongly that he would pardon Shaik should he become president," she said.

DA MP James Selfe said the party has petitioned the prison authorities for their reasons for releasing Shaik.

The DA made the application in terms of the Promotion of Access to Information Act, and did so because it believed the parole board had a duty to demonstrate that Shaik did indeed qualify for medical parole, he said.

"Without necessarily publishing the specific details of his medical condition, or jeopardising confidential information, we believe the parole board can and must demonstrate that Shaik does qualify for medical parole, under the very strict conditions specified in the Correctional Services Act of 1998."

Shaik was given parole after serving only 28 months of a 15-year sentence for fraud and corruption linked to South Africa's multi-billion rand arms deal. He spent most of that time in hospital.

The department of correctional services has said its decision was based on his deteriorating medical condition while the Shaik family has described him as "gravely ill".

Neither has revealed the nature of his illness.

Selfe said there was enormous public interest in Shaik's case, and a failure by the prison authorities to be more transparent would add to suspicion that he was freed on parole as a political favour.

"It will only cement the perception that this decision was politically-motivated, and taken to protect ... Zuma, given that Shaik had the potential to provide damning testimony when Mr Zuma appears in court later this year to answer charges of fraud and corruption."

Zuma's trial for corruption linked to the arms deal has been remanded to August 25. He is expected to file an application for a permanent stay of prosecution.