Deliberately inflicted head injuries were the cause of death of an 18-month-old South Waikato girl, police say.

Police launched a homicide investigation after Comfort Joy Witeri-Thompson died late on Tuesday night at Waikato Hospital and a post-mortem is underway.

Detective Senior Sergeant Matt Cranshaw, Taupo Area police said they believed Comfort died "as a result of head injuries police believe were deliberately inflicted."

"The inquiry team seeks the continued co-operation of family and close associates in bringing those responsible to account.

"As police seek to build up a picture of Comfort's life before she died, we welcome any information from her immediate family and their associates."

While Comfort's injuries were believed to be non-accidental, police were still identifying what occurred at the Tirau house prior to Comfort requiring medical assistance.

Police asked anyone with information relating to the girl's death to please contact Tokoroa Police on (07) 885 0100, or Crimestoppers, anonymously if necessary, on 0800 555 111.