A P-smoking mum has been charged with giving methamphetamine to her two daughters aged 3 and 6 in what experts claim is a legal first.

Adriene Johnson, 27, allegedly exposed her children to the highly addictive drug for at least 18 months.

The police case is laid out in court documents released to the Herald on Sunday.

Legal experts said the groundbreaking charges could provide police with a tougher means of dealing with drug-using parents.

Experts believed the children most likely inhaled the second-hand vapours as their mother smoked P in the house.

Police raided Johnson's North Shore home in March and found alleged evidence of drug use, court documents show.

They believed Johnson, 27, and her partner had been smoking P in the house while the girls were present.

The documents show the children were sent for health checks and lengths of hair were taken from each for analysis. Scientists allegedly determined that the girls had been exposed to P for at least 18 months.

Police raided Johnson's home again earlier this month, arresting and charging her with two counts of administering a class A drug to her children, as well as possession for a purpose.

Otago University law professor Mark Henaghan said police had long been grappling with how to deal with drug-using parents in a way that matched the offence.

"I don't think there's been a case like this before," Henaghan said. "This is a novel way of testing the drug laws."

A charge of administering or supplying drugs to the minor might more accurately reflect the harm done, he said. Administering a Class A drug carries a maximum penalty of life imprisonment.

Court documents stated that when Johnson was questioned over the charges she said she had never smoked P in the same room as the children "because it was not good for them to see and it was a bad drug".

Court records stated Johnson was on bail and expected back at North Shore District Court this week. Child, Youth and Family operations head Marama Wiki said the girls were in the care of family members and were safe and well.

"We are working with the family to provide support."