South Africa has issued an arrest warrant for Zimbabwe's former first lady Grace Mugabe over an alleged assault in an upmarket district of Johannesburg last year, a police spokesman has said.

After the alleged assault with an electric cable came to light in August 2017, the South African government granted Mrs Mugabe diplomatic immunity.

The immunity was overturned by a South African court earlier this year after the alleged victim, model Gabriella Engels, challenged the decision.

"I can confirm that a warrant for the arrest of Grace Mugabe was issued last Thursday," South African Police Service spokesman Vishnu Naidoo said, adding that police were seeking Interpol's help to enforce the warrant.

There was no immediate comment from Mrs Mugabe or from authorities in Harare.

Mrs Mugabe was seen as a potential successor to her 94-year-old husband Robert, who ruled Zimbabwe from 1980 until he was ousted in a coup late last year, at the time of the incident in a luxury hotel in Johannesburg's Sandton district.

She has denied the alleged assault, saying an "intoxicated and unhinged" Ms Engels attacked her with a knife after the model had gone to see the Mugabes' sons at the hotel.

South African advocacy group AfriForum, which represented Ms Engels, has dismissed Mrs Mugabe's denial as a lie.