The 10-month-old boy was brought unconscious to Te Kuiti Hospital in Waikato on Saturday afternoon.
"Tragically, despite the best efforts of medical personnel, he was unable to be resuscitated," Detective Inspector Graham Pitkethley said.
Police launched a homicide inquiry and were examining the property as part of their investigation.
A neighbour of the house where the baby lived told RNZ she heard a lot of screaming on Saturday afternoon.
The woman says the baby was taken to hospital after 2pm.
The neighbour said the baby's parents, a man and woman, rented the house and moved into the street before the baby was born but kept to themselves.
Oranga Tamariki said it was notified about the death of the baby boy.
Oranga Tamariki is assisting police in trying to understand the circumstances that led to this, deputy chief executive, service delivery Rachel Leota said in a statement.
"However, because this is a police-led investigation we are unable to comment further at this time."
Pitkethley said the homicide investigation is in its early stages.
"An initial examination showed the baby boy suffered violent, blunt-force trauma.
"We believe these injuries were not accidental.
"There is a lot of work ahead of us as we gather information."
The full extent of the baby's injuries would be determined by a post-mortem examination, but the results would not be known for some time, Pitkethley said.
Police would not be releasing details of his specific injuries at this time.
Pitkethley said officers had been speaking to members of the boy's family, including his parents.
"They are working with us as we establish what occurred in this young child's life and how he came to be so badly injured."
Police would be conducting a scene examination at a Te Kuiti address over the coming days, he said.
That property had been under police guard since Saturday afternoon.
"Te Kuiti locals can expect to see police in the town making enquiries and we ask for the public's co-operation and understanding.
"The death of any child is a shock to the community, and we know many in Te Kuiti will be affected by this.
"The public can help by talking with us; we need to hear from anyone who has information that could help the investigation."
The death was "a huge shock to the community", Waitomo District mayor John Robertson told Morning Report.
"We have our tragedies from time to time, but no community wants to see a 10-month-old baby apparently suffer like this baby did."
He did not have any more details, except to say police were speaking with members of the community.
Te Kuiti had a population of 4500 people, he said.
"A lot of sympathy goes to those who knew the child but shock at the fact that this could happen in our community."
Anyone with information was asked to call police on 105 and reference file number 240608/8263.
