A homicide inquiry has been launched after a six-month-old Ngaruawahia baby, admitted to Waikato Hospital earlier this week with apparent non-accidental injuries, died this afternoon.

Serenity Jay Scott-Dinnington was transferred to Starship Hospital on Tuesday night, and had been on life support since then.

Detective Senior Sergeant Mark Greene, officer in charge of the police investigation into the matter, said the baby died at about 4.35pm after life support was turned off.

"As a result, the police investigation has now been upgraded to a homicide inquiry."

A post-mortem will be undertaken in Auckland tomorrow to determine the cause of death.

Greene said a scene examination at the Ngaruawahia house where the baby had been living was underway.

"Obviously this is a sad development in this case. Our focus is very much on determining the events leading up to the baby's admission to hospital and her subsequent death."

Child, Youth and Family's head of operations, Jo Ann Field, yesterday confirmed the family was known to the agency before the incident.

"CYF is working with police to understand the events that led to this little girl being injured."

Serenity's older brother Travis, 4, was taken into emergency care where he was now "safe and well" with a CYF caregiver, she said.