A homicide investigation has been launched after a 6-month-old baby girl died after sustaining head injuries in Wellington.

The baby was taken to Wellington Hospital from her Raumati Beach home at about 4.30pm on Monday after sustaining the "severe" head injury, said a statement from police.

She died in hospital at 8.30pm last night.

The statement said a Kapiti man has been charged with one count of assault on a child and a homicide investigation has been launched.

Detective Senior Sergeant Grant Ferguson said police are talking to everyone who had contact with the baby prior to her admission to hospital.

"We want to build a full picture of the circumstances which led to emergency services being called to the address," he said.

"A post-mortem examination is due to take place later today which we expect will provide us with further information to progress our enquiry.

"Police are keeping an open mind about the involvement of other people, and we cannot rule out further charges being laid as our enquiry continues.

"This is a tragic death of a young baby and the investigation team is very focussed on establishing exactly what happened to her."

Forensic specialists were continuing to examine the scene at the Raumati Beach address today.

A further scene examination at a Waikanae address was completed last night.

The identity of the baby girl will be released once all next of kin have been informed.