"Child abuse is family violence in these circumstances, and it is clear this baby and its mother needed help."
Mr McDonald said the baby did not have a voice and police needed to act to support the mother and baby. The woman was sentenced to six months' supervision.
"People often believe drug-related activities are victimless, but it affects the people around them," Mr McDonald said.
He said adults making bad choices affected the most vulnerable in our community, and it was not okay to affect children by those choices.
Mr McDonald said the woman did not face any other charges, however, her partner faced other charges relating to the drug raid.
Warren Brookbanks, a professor of law at the University of Auckland, said the case was unusual and was almost certainly the only one of its kind in New Zealand.
He said the case was unlikely to have been successful if the woman had not pleaded guilty.
Professor Brookbanks said it would be difficult to prove that the woman had deliberately administered cannabis to her baby through her breast milk.
"It would be necessary for the prosecution to prove that the mother both knew she had cannabis in her system, and that she intended to administer it to the baby.
"In the absence of both of those mental elements, it would be impossible to prove a relevant administration of the drug," Professor Brookbanks said.
He said a person could not be convicted of administering a drug where they did not know they had consumed cannabis or did not know that cannabis in the system could be passed on to a dependent child through breast milk.
Allison Jamieson, Plunket's clinical adviser, said her organisation advised mothers against using cannabis, tobacco and other drugs.
"Most drugs, including cannabis and P, pass through the breast milk and are known to affect babies.
"It is safest for breastfeeding mothers to avoid smoking and taking drugs, because the long-term effects of this on the baby are unknown," Ms Jamieson said.