South Africa's National Prosecuting Authority (NPA) has no reason not to prosecute former Zimbabwean first lady Grace Mugabe after her diplomatic immunity was set aside at the South Gauteng High Court on Monday, lobby group Afriforum said.

"If the NPA still sleeps and still doesn't do anything, advocate Gerrie Nel is ready to carry on with the private prosecution on behalf of miss Gabriella Engels," spokesperson Willie Spies said.

On Monday, Judge Bashier Vally said that the decision to grant diplomatic immunity to Mugabe had been unconstitutional. 

He also ordered the international relations and cooperation minister to pay the costs.

Afriforum and the Democratic Alliance (DA) brought the matter to court after Mugabe was granted diplomatic immunity and left without standing trial for the alleged assault of Engels.

The DA federal council chairperson James Selfe said that Mugabe was granted immunity simply to shield her from being tried in a court of law and the judgment was a clear indication that no one was above the law. 

"That is very important because it means in future if somebody else comes into this country and tries to do what Mira Mugabe did, there's already case law in our books that says it's not applicable in South Africa," he said.

"This proves that no matter how exulted you are, you are subject to the same rule of laws as anybody else."

He said a warrant of arrest for Mugabe must be reissued and the government must ensure that she returns to South Africa to answer for her actions.

The case stems from an incident at a Sandton hotel where  Mugabe allegedly assaulted South African model Gabriella Engels after finding her in the company of her sons Robert Jr and Chatunga Bellarmine. 

Mugabe insisted that she was acting on self defence against Engels.