Grace Mugabe, the wife of Zimbabwean President Robert Mugabe has returned to her homeland from South Africa after failing to turn herself in to South African police to face charges of assaulting a woman in Johannesburg.

"Yes, she is back in the country. We don't know where this issue of assault charges is coming from," said a senior Zimbabwean government official, who declined to be named because they were not authorised to speak to the media.

Earlier, South African police were negotiating with Ms Mugabe's lawyers to turn herself in to face the assault charges.

South African police minister Fikile Mbalula had said the Zimbabwean first lady had already handed herself in to police and would appear in court shortly, but that later appeared to have been premature.

Twenty-year-old Gabriella Engels, who works as a model, told South African media Ms Mugabe had attacked her while she was visiting the Mugabes' sons Robert and Chatunga at a hotel in Johannesburg's upmarket Sandton district on Sunday.

There was no immediate public comment on the case from Ms Mugabe or her aides.

Confusion arose this afternoon when the magistrates' court where Ms Mugabe had been expected to be formally charged closed for the day without her appearing.

"The negotiations for her to hand herself in are still going on. We are at a point where we cannot effect an arrest yet," the police source said. "We will cross that bridge when we get there."

The source said that the 52-year-old had earlier agreed to hand herself over this morning but failed to do so.

Asked if Ms Mugabe was now considered a fugitive, the source said that was not the case at this stage.

The source said police were investigating a charge of assault with intent to do grievous bodily harm.

Another source in South Africa's prosecuting authority said Ms Mugabe had breached an agreement to surrender herself ahead of an expected court hearing this afternoon.

"They [the police] had an agreement that she hand herself over, but she never did," the source said.

South African media said Ms Mugabe was in the country to have an injured foot examined. It was unclear whether she was travelling on a diplomatic passport.

Asked whether she could be arrested despite having diplomatic immunity, Mr Mbalula said: "All those implications will be taken into consideration." He then added: "She will be charged."

A Zimbabwean intelligence source said Ms Mugabe had been travelling on an ordinary non-diplomatic passport and was in South Africa on personal business.

The News24 website quoted Ms Engels' version of events in the hotel room. "When Grace entered, I had no idea who she was. She walked in with an extension cord and just started beating me with it," the model said.

"She flipped and just kept beating me with the plug. Over and over. I had no idea what was going on. I was surprised ... I needed to crawl out of the room before I could run away."

News24 published a picture of what it said was Engels with a large gash in her forehead. "I am a model, with this scar over my face my whole career is ruined," she said.

It was unclear what triggered the incident.

Ms Engels told Talk Radio 702: "She just completely lost it. I was hit all over my body. I have bruises all over my body... I have two open wounds at the back of my head as well."

Ms Mugabe's two sons were kicked out of the Regent luxury apartment complex in Sandton last month after an incident in the middle of the night, staff at the complex told Reuters.

Regent manager Imelda Fincham did not elaborate but confirmed the pair had left. "They're no longer here," she said.

In 2009, a press photographer in Hong Kong said Ms Mugabe and her bodyguard had assaulted him.

Police there said the incident was reported but that no charges were brought.

President Mugabe, 93, spoke at a public event marking Defence Forces Day in Harare this afternoon, but did not mention his wife.

She was in the news in late July when she challenged her husband to name his preferred successor as president.

The issue of who will succeed Mr Mugabe has deeply divided Zimbabwe's ruling ZANU-PF party.

One faction supports Ms Mugabe and the other Vice President Emmerson Mnangagwa, who is recovering in a South African hospital after he fell ill and was airlifted from Zimbabwe.