Two-year-old Ruthless-Empire Ahipene-Wall's death is being treated as a homicide, with post-mortem results suggesting he died from multiple blows to his body.
Police are in contact with three family members who are persons of interest, and say wet wipes did not cause his death as some rumours have suggested.
Ahipene-Wall was taken to Hutt Hospital about 10.30am on Sunday in an unresponsive state. Efforts to revive him were unsuccessful.
Detective Inspector Nick Pritchard of Wellington Police said after investigation, the boy's injuries were believed to have occurred up to 12 hours beforehand.
The boy would have turned two years old on Wednesday.
"Whānau had been preparing to celebrate his birthday. He was an innocent child who should have been safe and loved, and should not have died as he did".
He said post-mortem results confirmed the baby's cause of death was from blunt force trauma.
"We believe that these injuries were not accidental. We are aware of reports that Ru had ingested wet wipes before his death, however the post-mortem confirmed that this did not contribute to his death. It was the injuries sustained that did."
Three adults he described as "persons of interest" were being actively engaged with by police but were not in custody, Pritchard said. One was a blood relative and the other two were wider whānau members.
Staff were actively working to engage with them as well as other whānau members.
"This is a heartbreaking situation and we have an extensive investigation team committed to getting answers about what happened to baby Ru.
"This level of violence toward a child is difficult to fathom and we will work hard to ensure a thorough investigation is carried out to determine exactly what happened. We urge those who have information about what happened to baby Ru to look deep and do the right thing and tell us what you know."
He said police were getting "varying degrees of engagement" with the three people of interest, and were encouraging people to come forward with all the information they had about what happened at the address on Sunday.
"Police are still gathering evidence, there's a lot of inquiry work still to do."
It was not until after the post mortem and scene examination completed at an address in Taitā on Thursday that police came to the conclusion it was a homicide, Pritchard said.
He said those thought to have been present when Ru was injured had been spoken to.
Detective Inspector Nick Pritchard. Photo: RNZ / Samuel Rillstone
Some discarded wet wipes were found at the scene, but police had no direct evidence Ru ingested them, Pritchard said.
"They were not a contributor to his death ... Ru suffered a number of injuries to his body ... at this stage I can't comment on the nature of what caused the injuries other than there were several to the external of his body."
Pritchard would not comment on what was being shared on social media: "It's part of society and it's not always going to be accurate".
Police would work to determine whether items were taken from the scene in the coming days, he said.
Police were also looking into the background of all the individuals concerned, and were working with Oranga Tamariki and other family members to understand the history of Ru and the family, he said.
Victim support had been engaged with the family since the death.
Photo: RNZ / Patrice Allen