Amber Heard says she punched Johnny Depp because she feared he would push her sister downstairs, as she alleged he had done to model Kate Moss.
Actor Heard, 34, is giving evidence in London’s High Court on behalf of the Sun newspaper, whose publisher News Group Newspapers is being sued by her former husband Depp.
Depp is suing for libel over a 2018 article that labelled him a “wife beater”.
Heard says the 57-year-old Pirates of the Caribbean star attacked her on at least 14 occasions and threatened many times to kill her.
He slapped, punched, head-butted and kicked her during violent, jealous outbursts brought on by excessive drinking or drug-taking, Heard says.
Depp denies hitting Heard, and says the claims are a hoax and that he was the victim of her assaults.
On her second day in the witness box on Tuesday, Heard was questioned about an incident in March 2015 when she hit Depp during an argument at their Los Angeles penthouse, saying it was the first time she stood up to him.
She told the court she believed Depp was about to push her sister Whitney down the stairs.
“He was about to push her down the stairs and the moment that happened I remembered information I had heard (that) he pushed a former girlfriend – I believe it was Kate Moss – down the stairs,” Heard told the court.
She said that memory had come to her in “a flash” and she reacted to defend her sister.
Depp’s lawyer Eleanor Laws said this was the first time Heard had mentioned any recollection of the alleged Moss incident, saying it had not appeared in any of her previous statements or legal declarations.
“I have never changed my story,” Heard said, but agreed she had not said it before.
Heard has said she was repeatedly assaulted by Depp during their stay in Australia.
She described how at one stage, he had forced her, naked, against a kitchen counter covered in glass and pressed on her neck, choking her.
Holding back tears, she told the court she tried to push him off, saying “Johnny stop, it’s me, stop please”.
As she gasped for breath and writhed around, she said she cut her feet and arms on the glass.
However, Laws said a picture of her wounds caused in the incident looked like self-harm scars.
“Those scars on my arm were from that evening, I have never self-harmed,” Heard said.
Asked why she agreed to return with Depp to the same house in Australia a couple of months later, she said: “Johnny – the monster – nearly killed me. I loved the Johnny who was clean and sober.”
Laws said that in mid-2015, Heard was in a relationship with Tesla chief Elon Musk.
She denied she had an affair with Musk, or anyone else, while she was married to Depp.
Heard also said she had only ever acted in self-defence, saying if she contradicted Depp he would become violent.