The Democratic Alliance (DA) has applied for direct access to the Constitutional Court to have the diplomatic immunity granted to Zimbabwe's first lady Grace Mugabe set aside.

The party filed papers this week arguing International Relations and Co-operation Minister Maite Nkoana-Mashabane's decision to let Mugabe off the hook for alleged assault was "wholly without legal merit", and should be declared invalid.

Mugabe was accused of beating South African model Gabriella Engels, 20, with an extension cord in Sandton on August 13, during a visit with her two sons who are staying in Johannesburg.

"The DA believes that the decision by the minister, in granting immunity, was hasty, embarrassing and above all, illegal and unconstitutional," DA federal council chairperson James Selfe said.

"It is frankly unconscionable that after the scathing ruling by the Constitutional Court in the [Sudan President Omar] Al-Bashir matter, that the ANC-led government would once again let a high profile person escape justice in South Africa."

It was clear that Mugabe was granted immunity simply to shield her from being tried in a court of law for her assault on Engels and two others, Selfe argued.

Condoning such behaviour, as the granting of immunity did, could not possibly be in the interest of South Africa. "There is therefore no legal basis for such a decision."

Grace Mugabe was not a member of the Zimbabwean government and was in South Africa on personal business, he continued.

There was nothing in either South African or international law which rendered her deserving of diplomatic immunity, and was an example of an abuse of statutory powers at the expense of citizens.

Engels, President Jacob Zuma, Nkoana-Mashabane and National Director of Public Prosecutions Shaun Abrahams have all been listed as respondents.

The DA has also applied for costs to be sought, and said the "precedent-setting nature" of the minister's decision required direct access to the highest court in the land, and their employment of two counsel.

Nkoana-Mashabane this week said her decision was "painful" and "agonising". That has not spared her receiving widespread criticism in civil society.

The portfolio committee on international relations and co-operation resolved on Wednesday to invite the minister to explain her decision in Parliament at the soonest possible date.

Mugabe returned back to Zimbabwe on Sunday.