The man who killed a 6-month-old girl struck out with "back-hand blow" to the side of the baby's head.
Gracie May McSorley died in July last year despite her mother, Charlotte McSorley, desperately attempting CPR.
Michael Te Kouarehu Kereopa, 32, today pleaded guilty in the Wellington High Court to Gracie's manslaughter.
A summary of Kereopa's offending has now been released by Justice Rebecca Ellis.
It describes Kereopa being left alone with Gracie for 45 minutes while Charlotte McSorley went to collect her oldest daughter on July 6.
While she was away from her Kapiti Coast home, Gracie was "crying and unsettled".
Kereopa took her out of her cot, trying to comfort her. But he couldn't get her to settle.
"He has become frustrated and lost his temper with the victim, lashing out and causing a severe head injury with a back-hand blow, striking her to the right side of the head," the summary says.
"Medical experts have agreed that this is a high-velocity injury, similar to an injury you would sustain in a high-speed motor vehicle collision".
The injury was "non-accidental" and caused by Kereopa lashing out in frustration.
Shortly before Charlotte and her other daughter returned, Gracie began vomiting in her cot.
Kereopa got her out and held her over a tea towel placed on the floor, while Gracie continued to vomit.
"The victim's body was limp and her eyes were rolling around in her head," the summary says.
Kereopa put Gracie back in her cot and she appeared to be sleeping on her stomach. He then told Charlotte the baby's breathing turned "funny" and she had vomited.
Charlotte checked on her but she didn't have a temperature and appeared okay, so she tucked her daughter into a blanket and left her to sleep.
Meanwhile, Kereopa set up a laptop for the other child to use and went outside to stack firewood.
About 45 minutes after arriving home, Charlotte McSorley again checked on her baby and noticed she wasn't breathing normally and her eyes and lips were puffy.
When picking her up, Charlotte saw bruising inside and behind her right ear as the baby started convulsing in her arms.
Gracie was put on a bed but wouldn't open her eyes. She was floppy and non-responsive.
An ambulance was called and Charlotte performed CPR until paramedics arrived.
Gracie was flown to Wellington Hospital by helicopter and despite surgery to remove a blood clot on her brain and a section of her skull to relieve swelling died the next evening, shortly after she was taken off life support.
When spoken to by police, Kereopa admitted being alone with Gracie for 45 minutes and initially claimed the baby had fallen and banged her head on a fireplace hearth, where she'd been sitting, propped up by pillows.
He then changed his story, saying he'd dropped Gracie on the hearth when he picked her up.
He also said he shook Gracie because he was angry she wouldn't stop crying.
Police consulted medical experts who said Kereopa's explanations didn't accord with the injuries.
He had been in a relationship with Charlotte for about eight weeks and had recently moved into her home, a house owned by her mother.
In the High Court at Wellington today, Kereopa was convicted and given a first warning under three-strikes legislation.
He was initially charged with assaulting a child and then murder, before the manslaughter charge was laid today.
"This is yet another sad reminder of New Zealand's unacceptable levels of child abuse," a police statement said after Gracie's death.
"We must all work together to keep our children safe and free from harm. Now a family is mourning the loss of a beautiful little girl at the hands of someone she should have been able to trust to care for her."
Kereopa will stay in custody until sentencing next month.