They say the boy was taken to Whangarei Hospital by his mother on Friday morning with significant head injuries.
He was airlifted to Starship Hospital in Auckland but died just before 11pm.
A police spokesperson says 25 staff are working on the investigation and have been speaking to the baby's family.
They are making a forensic examination of an address in the suburb of Kamo. A post mortem is being carried out on Saturday.
Police say the boy's mother and her partner live at the address.