|President Robert Mugabe and his wife Grace attend a rally in Chinhoyi|
South Africa's government granted diplomatic immunity to her on Saturday despite strong objections from the opposition.
"I hereby recognise the immunities and privileges of the First Lady of the Republic of Zimbabwe, Dr Grace Mugabe," the international relations minister, Maite Nkoana-Mashabane, said in a notice published in the government gazette on Sunday.
Police had previously put out a red alert to stop Mrs Mugabe, 52, leaving the country.
Gabriella Engels has accused Grace Mugabe of assaulting her while she was visiting the first lady's sons in a hotel room in an upmarket suburb of Johannesburg.
The 20-year-old claims the first lady's bodyguards stood and watched as Mrs Mugabe hit her with an extension cord.
On Sunday, state media reported Mrs Mugabe had arrived in Harare with the 93-year-old President, Robert Mugabe, aboard an Air Zimbabwe plane.
There has been no official comment from Harare on the allegations.
South Africa's main opposition group, the Democratic Alliance, is demanding an immediate parliamentary inquiry.
It has accused the government of "complicity" in allowing her to "flee the country in the dead of night to escape criminal prosecution".