A P-smoking mother has been convicted of giving her young daughters methamphetamine through second-hand smoke inhalation, in what experts are calling a legal first.

Child Youth and Family took the girls, aged 3 and 6, from their parents and put them in the care of family members.

Experts say the conviction paves the way for police to use the serious charge against parents in future.

Adriene Johnson, 27, has pleaded guilty to two counts of administering the highly addictive drug to her children and one of possession for a purpose.

Johnson was arrested in a police raid on her North Shore home, but her partner and father of the girls fled.

He is still on the run and arrest warrants have been issued for him. It is likely that, when captured, he will face the same charge. Johnson is due to be sentenced in Auckland District Court next month.

Child, Youth and Family operations head Marama Wiki said the girls were in the care of family members and were safe and well.

Samples of the children's hair that were sent for scientific analysis revealed they had been exposed to P for at least 18 months before the October police raid. The charges were reported by the Herald on Sunday that month.

New Zealand Drug Detection Agency director of communications Nick Mcleay commended the police for taking the unusual step. Charges traditionally used in child P cases did not accurately reflect the seriousness of the offence, he said.

A charge of administering a class-A drug to a minor, however, carried a maximum penalty of life in prison. " It will be interesting to see whether [the courts] use the full force to send a deterrent message," Mcleay said. "It will be good to see them give a decent sentence to show it's unacceptable."

Children exposed to P, particularly when young, can suffer developmental problems and other chronic illness.

Otago University law professor Mark Henaghan said Johnson's guilty plea meant the legal arguments behind the charges were not heard and had not been tested to form precedent.

However, the conviction did allow police to lay the serious charge in future cases, something they would likely take full advantage of.