Police have named the 16-month old who died overnight from non-accidental injuries and said they were speaking to the three occupants of the house where he lived.
Riley Justin Osborne was airlifted last Friday to Auckland's Starship Hospital from his Kerikeri home.
Police said he suffered brain injuriesand been left blind.
He underwent emergency surgery but, in spite of best specialist medical care, died last night.
Police said today they were talking to the victim's 22-year-old mother, 21-year-old stepfather, and woman family friend who all lived at the Cobham Road address. They were also keen to speak to any member of the public who had contact with the trio over the past few days.
A post mortem on the boy was being undertaken in Auckland this afternoon.
Police said the house where the boy lived was often the scene of noisy, all-night parties to which officers had been called.
Police spokeswoman Sarah Kennett said a team of 17 officers were investigating the case.
The boy's mother and other relatives were believed to have been at the boy's hospital bedside when he died.
On Monday, the Herald reported medical experts saying the boy would have been left blind and have permanent brain damage.
The case comes after the Herald ran a series investigating child abuse in New Zealand.
The Our Lost Children series spurred businesses and individuals to donate to groups trying to prevent child abuse and help its victims.
Starship hospital pediatrician Dr Patrick Kelly, who has given evidence in many child abuse cases, said at the time that about one in five babies with serious head injuries died.
Of the 80 per cent who did not die, he estimated about a third would have permanent, severe damage.
Another third would suffer moderate damage.
At Starship, hospital, where many of the most seriously injured children are treated, 86 children were treated between 2001 and 2007 for non-accidental head injuries.
About 45 new cases of child abuse come to the attention of social workers at CYF every day.
