Initially she denied it, Crown prosecutor Yelena Yelavich said, but after repeated questioning she finally confessed.
The defendant grabbed a brick on which he told the girl to place her hand; saying she would do so if she wanted them to stay together.
"So I did; stupid thing to do but I did," the teen said in a video interview played for the jury.
He allegedly smashed her right index finger, which hospital staff later that night found, fractured it.
The complainant spent the next two days in hospital where she underwent surgery to realign the bone.
When she was discharged she returned to the defendant's south Auckland home, "because she wanted to make the relationship work", Ms Yelavich said.
"Her return was met with further accusations and further punishment."
On December 23, it is alleged the man bound her hands and feet with rope, slapped and punched her around the head and attempted to suffocate her to the point where blood vessels in her eyes burst.
The Crown said the defendant told her to open her mouth, threatening further beatings with the hammer if she did not.
When she did, he allegedly took to her teeth with a pair of pliers.
"Despite persistent attempts he was not able to do that but did chip and damage the teeth," Ms Yelavich said.
Before going to sleep he allegedly urinated on the girl.
The prosecutor said the abuse continued on Christmas Eve when he both violated and threatened her with a gun.
It is alleged the man also punched her genitalia and stubbed out a cigarette on it.
Earlier that day he went to Bunnings to buy cable ties and duct tape and returned telling the teen "Merry Christmas", she said.
On Christmas Day the children's mother came to pick them up and the complainant took the opportunity to escape the property.
The defendant tried to drag her back, the Crown said, but she fled to a nearby house where they gave her fresh clothes before taking her to a friend's house.
Defence lawyer Mark Ryan warned the jury of "prosecutorial prejudice".
"Just because he's been charged by the police and put on trial before you, does not mean to say he's guilty," he said.
The defence to the first hammer blow was that the girl "was a willing participant and effectively encouraged him to do it".
As for the rest of the charges, Mr Ryan said they were invented by the complainant in a bid to escape charges against her for molesting the man's young daughter.
The trial is expected to last two weeks.
CHARGES
• 4x Threatening to do GBH
• 3x Injuring with intent to injure
• 3x Indecent assault
• 2x Wounding with intent to injure
• 2x Kidnapping
• 1x Sexual violation by unlawful sexual connection
• 1x Assault
• 1x Assault with a weapon
