The police are being criticised for not giving top priority to a 111 call where a two-year-old boy had reportedly been punched in the face by a gang member who was holding his mother against her will.
The Independent Police Conduct Authority has released its findings into two incidents in the Auckland suburb of Mount Wellington in 2013.
It outlines one of the incidents where a woman rang 111 to say her daughter and her child were being held by gang members.
But there were no police units available to attend and the call was logged as one level down from top priority.
That meant the same effort was not made to find officers to attend immediately.
The grandmother eventually went to the house and was able to get her daughter and grandson to Starship Hospital where they eventually spoke to police, several hours after the incident.
In its report, the Authority said police initially failed to recognise the seriousness of a reportedly violent incident involving a small child.
It said the call should have been given top priority because the safety of a child was at stake, and police failed in some aspects of the subsequent investigation, partly because of communication failures between two districts, Auckland City and Counties Manukau.
It said there was a question mark over whether the woman's story about how her child was injured was true, but a proper investigation into domestic abuse was not launched.
By the time the police Child Protection Team conducted a scene examination at the woman's house, nearly two weeks after the gang incident, she had moved out and it had been thoroughly cleaned.
Officers conducted the examination anyway.
Police said they accepted the findings and were disappointed they let the boy and his family down.
They said they had already started giving better training to frontline staff.