"People often believe drug-related activities are victimless, but it affects the people around them."
Adults making bad choices affected the most vulnerable in the community, and it was not okay to affect children by those choices, Mr McDonald said.
"We need to take the children's needs into consideration."
University of Auckland law Professor Warren Brookbanks said the case was almost certainly the only one of its kind in New Zealand.
It was unlikely to have been successful if the woman had not pleaded guilty because it would be very 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.
Plunket clinical adviser Allison Jamieson 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 breast-feeding mothers to avoid smoking and taking drugs because the long-term effects of this on the baby are unknown," she said.
* If anyone has concerns about the welfare of children, they can call 0508 FAMILY.
