Amber Heard has admitted punching her ex-husband Johnny Depp, saying it was to prevent him from pushing her sister down a flight of stairs and claiming he had previously done this to his ex-girlfriend Kate Moss.

Giving evidence for a second day at the high court in London, Heard recalled an incident in March 2015 in the couple’s Los Angeles penthouse when she struck Depp because, she said, she wanted to protect her younger sister Whitney.

Depp, 57, is suing the Sun’s publisher, News Group Newspapers (NGN), and its executive editor, Dan Wootton, over an article that called him a “wife beater” and referred to “overwhelming evidence” that he had attacked Heard.

Depp denies ever hitting Heard, 34, who has submitted details of 14 occasions during their relationship when she claims he assaulted her.

Referring to the March 2015 fight, Heard told the court: “I did strike Johnny that day, in defence of my sister.” She said Whitney was standing at the top of a staircase in their apartment. “I had heard that he had pushed a former girlfriend, I think it was Kate Moss, down the stairs. And I thought of that at the moment and I reacted in defence of her [Whitney].”

She said the memory had come to her in “a flash” and she had reacted instinctively to defend her sister.

Depp’s lawyer, Eleanor Laws QC, said it was the first time Heard had mentioned any recollection of the alleged incident involving Kate Moss. “I have never changed my story,” Heard countered, but she agreed she had not referred to it before.

Laws also challenged Heard’s account of Depp allegedly grabbing her hair with one hand and punching her with the other during the clash. The barrister said Depp’s hand was in a plaster cast at that time.

Laws said: “You are just making this up as you go along.” Heard replied: “I didn’t include every detail or every thought.”

Heard was asked about an incident when the couple were in Australia and she alleges Depp attacked her in what she described as “a three-day hostage situation”.

Laws put it to Heard that the scars on her arm were not from the incident but were “self-harm” injuries. Heard said: “I have had the scars since that evening. Johnny’s the self-harmer; I’m not a self-harmer.”

Earlier, Heard insisted she did have visible injuries on her face on 21 May 2016 after Depp allegedly assaulted her in their LA penthouse and their marriage finally fell apart.

Asked by Laws why staff who lived in their building and Depp’s friends had given evidence saying they could see no injuries, Heard said: “I can’t explain why Depp’s lifelong friends and employees [said that].”

Heard said she had been wearing makeup, which may have covered up the injuries in the days after the alleged violence. “Whenever I leave the house, I wear makeup,” she added.

The hearing was shown a video of Heard meeting the actor James Franco the day after the incident. “He saw my face and said: ‘Oh my God what’s happened to you,’” Heard said. “He said: ‘What the fuck?’”

In a statement made in February and released this week, Heard said: “While these proceedings are not brought against me personally, they are directed at the veracity of my allegation of domestic violence. I am astounded that years after our divorce agreement was concluded, I continue to have to answer his continued harassment and bullying, and I continue to be blamed by him [Depp] for a mess of his own making.

“Although I am not being sued personally in this claim, I feel that the action is very much aimed at me because Johnny is using it as a platform to repeat his false allegations that I lied about my experience of domestic violence in an attempt to address his perceived loss of his reputation at my expense, which I find very upsetting.”

She said she hid her injuries in the early part of their marriage. “Throughout the relationship, others noticed I had cuts and bruises,” she said in the statement.

“I made excuses. People asked me: ‘Is he hitting you?’ I would deflect it; I wasn’t sharing it with people because I was trying to protect him and our relationship – and to protect myself from the humiliation that I felt about anyone knowing that I would allow this to happen to me. I became an expert in covering up the bruises and injuries. Later in the relationship it became impossible to hide.”

Asked whether she had an affair with the businessman Elon Musk while she was married to Depp, Heard denied she had been involved with him until after she and Depp had broken up in 2016.

She has told the court that Depp accused her of having affairs with all her co-stars, including James Franco, Eddie Redmayne, Channing Tatum, Billy Bob Thornton and Kevin Costner. He had nicknames for those he considered a “sexual threat”, she added, such as “pumpkin head” for Leonardo DiCaprio and “potato head” for Tatum.

Heard denied she left excrement on her own bed on the day after their relationship ended or that any of her friends were responsible. “It was disgusting. I can’t fathom what adult would do such a thing. I don’t think it’s funny. I don’t think it’s a prank. I can’t imagine what human being would have a sense of humour like that other than Johnny,” she said.

Heard has suggested the mess may have come from their pet Yorkshire terrier Boo. The court earlier heard that Boo was not entirely house-trained and on one occasion ate a bag of Depp’s cannabis that was left lying around.

The hearing continues.