The department of international relations and cooperation said it will study Monday's ruling by the high court in Johannesburg which set aside the decision to grant diplomatic immunity to former Zimbabwean first lady Grace Mugabe.

"The Department will study the judgement and comment later if necessary," the department said in a statement.

On Monday, Judge Bashier Vally said the decision to grant diplomatic immunity to Mugabe after she allegedly assaulted a South African woman had been unconstitutional. 

Vally also ordered the Minister of International Relations to pay the costs.

Mugabe allegedly assaulted model Gabriella Engels after she found her in the company of her two sons at a hotel in August last year. Mugabe claimed she acted in self-defence.

She was allowed to leave the country without being prosecuted.

In May, the international relations department argued it did not grant Mugabe diplomatic immunity, but rather recognised it in terms of international law.

Federal council chairman for South Africa's main opposition party, the Democratic Alliance, James Selfe, disputed the department's rationale and said there was nothing in either South African or international law which rendered Mugabe deserving of diplomatic immunity.

Selfe said Mugabe was granted immunity simply to shield her from facing trial and that the decision had no basis in law and could not be in the interests of South Africans.

"What we got today is justice and that was very important because a very serious crime was committed against Miss Engels," he said.

Selfe 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.