Bringing the person who inflicted fatal non-accidental head injuries on a six-month-old baby girl this week to justice is "everyone's responsibility", police say.

Serenity-Jay Scott-Dinnington was admitted to hospital on Tuesday after an "incident" at the Havelock Rd house where she lived with her mother, Chelsea Scott, 21.

Her life support was switched off at Starship Hospital yesterday afternoon, and she died at 4.35pm surrounded by family members.

Police said a post-mortem examination was still being carried out in Auckland but confirmed the little girl's injuries were not accidental.

Officer in charge of the homicide investigation, Detective Senior Sergeant Mark Greene, appealed to anyone who knew something about the incident to come forward to police.

A lot of discussion had taken place in Ngaruawahia and around New Zealand since Serenity was first admitted to hospital - both in conversations and in online chatrooms - but it needed to be shared with investigators, he said.

"An innocent, six-month-old baby is dead. She has died at the hands of another and it everyone's responsibility to ensure that someone is held accountable for her death.

"If anyone knows anything about how Serenity sustained her fatal injuries, we are asking them to please come forward to tell us what they know."

Mr Greene said a scene examination at the house in Ngaruawahia where Serenity had been living leading up to her death was continuing today.

People could give information on the case via the anonymous police Crimestoppers line 0800 555 111, he said.

Police began their investigation on the day Serenity's injuries were inflicted.

While Miss Scott went with her daughter in the ambulance, her brother Drew Scott - who also lived at the house - and her fiance, Mathew Ellery, were taken to the Huntly police station for questioning.

It is understood Mr Ellery went to check on Serenity and yelled to Miss Scott for help.

Mr Scott also ran into the baby's room and after seeing her, believed she was dead.

Mr Scott could not be reached last night, but earlier told one media outlet that he was watching TV when he heard his sister scream "help, help".

The rest of the ordeal was a blur as Chelsea and Serenity left for the hospital, he said.

On her Facebook page, Ms Scott described her daughter as "so cute".

"Our lil angel," she wrote. "Im so proud of my little gurl! I was just told by a Plunket lady that Serenity is the youngest baby she knoz of 2 b crawlin around ... Shez a clever little gurl!"

Miss Scott's mother Celeste Scott sobbed yesterday when she heard that her granddaughter had died.

"I'm totally freaking out ... Oh my God, I can't believe she didn't make it to eight months old," she said. "I don't know what to think, to be honest."

Mrs Scott had not been able to reach her daughter.

She said it was shocking to think one of her family could be responsible for Serenity's death.

"Oh my God, it could be anyone. It's disgusting. I just want to know what the hell happened."

Mrs Scott said Serenity was a cute, quiet and "very mellow" baby.

"Chelsea was a good mum. She's always been a great mum to [her other child] Travis and Serenity."

CYF said on Wednesday that Serenity and her family were "known" to it but refused to give details of its involvement.

Serenity's 4-year-old brother Travis was now in CYF care.

"The injuries suffered by this baby girl are tragic and our thoughts are with her and those who love her," said CYF head of operations Jo Ann Field.

Jessica Dinnington, whose brother Zane was in a relationship with Miss Scott for about a year before she met Mr Ellery, told the Herald she was saddened by the events.

She said Serenity took her family name as well as Miss Scott's surname but a DNA test showed her brother was not her father.

"She's not Zane's, but he was the partner to her when she moved into Ngaruawahia," she said. "He stayed with her right through her pregnancy."

Figures released under the Official Information Act show 130 children were admitted to the Starship with injuries from "definite or probable" child abuse between July 2005 and July last year.

Including Serenity, 18 children have died in the Starship from such injuries.

Other high-profile cases include twins Chris and Cru Kahui and Nia Glassie.

The chief executive of advocacy group Child Matters, Anthea Simcock, said Serenity was the third child she knew of to die in similar circumstances this year.

"And that's three that didn't need to die."