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The girl said the sexual assaults began when she was as young as 7 and only stopped when her younger sister made allegations about the man.
"It was the worst thing in the world. I hated it so much,'' she said.
"It happened almost every time I was with him.''
The defendant - who has interim name suppression - is on trial facing 14 charges, including rapes, unlawful sexual connections and indecent assaults, against the two girls.
Eleven of the charges are representative, which means the Crown alleges there was at least one instance.
The first complainant's taped interview was played for the jury before the Dunedin District Court yesterday.
She recalled one incident that allegedly took place at her stepfather's workplace.
"He just put me on the desk and started trying to touch me and stuff. I tried to push him away but I was only little so I wasn't strong,'' the teen told police.
The girl also said she was abused in the bed the man shared with her mother.
"He would grab me and put me under him and rape me. He'd have sex with me and I'd be crying,'' she said.
"He said: `I love making love to you' and I wouldn't say anything, I'd just cry.''
The witness said she had tried to repress as many of the memories as possible but "some things I just can't get rid of as well as others''.
Interviewer Constable Tracey Bransgrove pressed the girl on how many times she had been raped.
"More than two? More than 10?'' she asked.
"More than 50 times, probably,'' the teenager said.
The alleged abuse began when the defendant moved in with the family in Dunedin but the girl said it continued when he moved to Mosgiel.
The sisters were preparing to spend the night at their stepfather's home in January 2015 when the reluctant older girl told her mother she had been inappropriately touched by him.
The court heard the woman went to the younger girl's bedroom where similar allegations were made.
Instead of going to the police about her daughters' claims, she went to her ex-partner's mother who was adamant he would not have committed the crimes.
It was some months later, when the younger complainant told a school friend, that police were finally contacted.
Counsel David More quizzed the older girl yesterday about why she had not spoken out sooner.
She had attended meetings with social workers and school counsellors without making the sexual allegations and Mr More suggested she had concocted the story.
"I'm not lying about this. It did happen. All of it happened,'' the girl said.
"I always wanted to but was too scared about what would happen. I was scared no-one would believe me and I was scared I was going to end up here doing something like this. I didn't want to talk about it, because no-one should have to,'' the teen said.
"Mum was still in love with him and I didn't want her heart to break.''
The younger complainant is expected to give evidence tomorrow.
The trial, before Judge Kevin Phillips, is expected to last at least a week.