A man has been charged with the murder of a two-month-old triplet who died in Whanganui Hospital in January.

A 29-year-old Whanganui man is due to appear in court in Tuesday after being arrested over the death of Hinekawa Topia on January 12.

Hinekawa wasn't breathing when she was taken to hospital by her parents and attempts to resuscitate her were unsuccessful.

Police launched a homicide investigation that day after she died.

A post-mortem examination revealed Hinekawa died of a non-accidental head injury. Her siblings were taken into the care of Child, Youth and Family.

Inquiry head Detective Senior Sergeant Neil Forlong said medical evidence was crucial in leading to Tuesday's arrest.

"A small team of staff have worked tirelessly on the inquiry and recently medical information came to hand that has ultimately resulted in this arrest," he said.