Police are investigating two separate cases in which babies were admitted to Waikato Hospital with non-accidental injuries on Tuesday.
They are looking into the case of a six-month-old girl who has been transferred to Auckland's Starship Hospital with critical head injuries.
The officer in charge of the investigation, Detective Senior Sergeant Mark Greene, says a scene examination has started at the girl's home in Ngaruawahia.
They say the girl's family is cooperating with inquiries, and that Child, Youth and Family is also involved.
Meanwhile, Tokoroa police are investigating the case of an eight-month-old girl also admitted to Waikato Hospital.
The baby is in a comfortable condition on Wednesday.