The girl was flown to Waikato Hospital and died late on Tuesday.

Police have launched a homicide investigation after an 18-month-old girl died of head injuries.

Comfort Joy Witeri-Thompson, from a South Waikato family, was airlifted from Tīrau to Waikato Hospital on Monday, July 23, with critical injuries.

She was put on life support and died in hospital late on Tuesday.

A post-mortem was to be conducted on Thursday.

Taupō police investigations manager Detective Senior Sergeant Matt Cranshaw confirmed police had launched a homicide investigation into the girl's death.

"These are serious non-accidental injuries that have caused her death.

"We would like to speak to anyone with any information in relation to this girl and her death," Cranshaw said.

"This is obviously a tragic event for the family and the small community of Tīrau."

Police were still trying to establish when the girl was injured, though it's believed it happened at her home in Tīrau, not far from the centre of town.

Family members sought medical help for the girl on Monday and an ambulance was called. She was then flown to Waikato Hospital by rescue helicopter.

The hospital notified police and a forensic examination had been completed at the girl's home.

The girl had been in the care of family and living with her siblings, Cranshaw said.

Oranga Tamariki Regional Manager Bay of Plenty, Tayelva Petley confirmed the organisation had been informed of Comfort's death.

"Our thoughts are with those who loved her. We are working with her whanau to ensure the safety of her siblings."

Petley was unable to comment further while a police investigation was underway.

Detectives were speaking to family members.

Police figures show that between 2007 and 2014, children aged under 15 were the victims of 76 homicides. Of those, 60 were younger than five.

Anyone who may have information is being asked to contact Tokoroa police on 07-885 0100. Information can also be left anonymously via Crimestoppers on 0800 555 111.

If you're worried about a child, you can contact Oranga Tamariki by freephone on 0508 326 459, or email: contact@ot.govt.nz. If the child or young person is in immediate danger, call police on 111.