A "small group of whānau" are refusing to help police investigating the death of a baby boy in South Auckland.

Officers investigating the homicide of the 10-month-old child are calling for people to speak up, as some within the family have moved to not co-operate.

It comes after the child's death on Sunday, September 6, after police were called to an incident at a property in Manurewa the day before.

Counties Manukau Police says a number of family members are not co-operating with police looking into the circumstances of the boy's death.

"There are a small group of whānau who are choosing to not co-operate with us as we investigate the death of this 10-month-old boy," Detective Senior Sergeant Kevin Tiernan said.

Police acknowledged that they had spoken to several other family members who had been working well with officers.

But Tiernan said it was disappointing others within the family were not.

Oranga Tamariki received anonymous report of concern before death

"This is disappointing for police and I urge anyone who is approached in this investigation to consider speaking with us so we can establish what exactly happened."

It has also been revealed that Oranga Tamariki received an anonymous report of concern in relation to the baby boy.

Deputy chief executive for Oranga Tamariki, Glynis Sandland, said they became involved shortly before the boy died.

"We were looking into these concerns and working with the family to gather further information," she said.

A number of other children who live at the property where the boy was fatally injured have also been removed.

"I can confirm the other children in the home are with wider family members and there are supports in place to keep them safe," Sandland said.

Tiernan said they were committed to holding whoever was responsible for the baby boy's death to account.

Anyone with information is asked to call the investigation team on (09) 261 1321, Police on 105 or CrimeStoppers anonymous on 0800 555 111.