This was attributed to probable accidental asphyxia by being overlain by her sibling.
Hawkes Bay Detective Sergeant Tim Smith said the death was preventable.
"In these cases we have a responsibility to investigate thoroughly and to consider any culpability on the part of the parent," he said.
"It is not our aim to revictimise parents after the tragic death of a child and any prosecution is taken after careful consideration of all the circumstances".
The dangers of babies co-sleeping with parents were well known and health agencies were proactive in their efforts to educate families about the issue. Mr Smith said in this case, the information provided to the parents by health professionals was ignored.
The child was at higher risk due to being born premature, of low birth weight, and having parents who were smokers, including the mother smoking during pregnancy.
"There is a responsibility on parents to ensure safe sleeping advice is adhered to but we would also like to see all parents, grandparents, whanau and family take responsibility for ensuring safety around co-sleeping," Mr Smith said.
"As parents we have social and legal obligations to ensure the safety of our children."
The mother and her partner had been given comprehensive information on co-sleeping and other risk factors when their baby was born premature.
A family safety plan had been prepared and the couple were given a pepi pod by health authorities to use in bed as there was such a concern about co-sleeping.
A pepi pod is a plastic box with a mattress, cover and bedding package, including a top sheet that wrapped the baby securely to keep them on their back while sleeping. They are designed for babies up to 6 months old.
The pepi pod was found leaning against the wall in the bedroom.
