More than 500 Northland children are being raised by their grandparents, and a trust responsible for supporting them says the main reason is methamphetamine use by their parents.

Grandparents Raising Grandchildren Trust chief executive Kate Bundle said drug use was the number one reason for a child to be in their grandparents' care, but it doesn't exist on its own, rather combining with other factors to cause a family breakdown.

Ms Bundle describes meth as a "prevalent cause" and "comorbid factor" to a child being cared for by their grandparent or grandparents.

"In the vast majority of cases, their story tells a story of P."

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Ms Bundle said the trust has 280 Northland families registered with them, and those families are raising around 530 children.

Figures on the Ministry of Social Development website show 418 Northland children were in state care as of December last year. Of those, 313 were living out of their home.

Ms Bundle said her figures would overlap with the ministry's but the exact overlap was unable to be established.

Mrs Bundle said not all of the children being raised by the families they are involved with have been had ministry involvement.

"From our experience a lot of grandparents step up and take the children into their care more informally."

The trust commissioned a report into grandparents raising grandchildren in NZ. Among the questions, it asked participants to list all the factors which contributed to the child ending up in their care. Drug addiction was cited as the factor the highest number of times.

The top six, which were often cited together, were drug use, domestic violence, family breakdown, neglect and parent unable to cope and alcohol abuse.

"Families where P is involved, it becomes the catalyst to a whole family breakdown," she said.

Ms Bundle said when parents used and became addicted to methamphetamine, they often neglected their children, and put them at risk of violence.

She said no other drugs, apart from meth, were cited in the report.

Ms Bundle said they were seeing an increase in families registering with them across the country, with numbers up 22 per cent in the past three years.

Oranga Tamariki Whangarei manager Claire Burnton told RNZ meth appeared to be on the rise in Whangarei.

"It's probably the single biggest factor in terms of children being exposed to harmful adult behaviour and therefore the need for Oranga Tamariki to become involved to create safety," she said.

Ministry for Vulnerable Children Oranga Tamariki chief executive Grainne Moss said methamphetamine was not the only issue.

"In almost every case there are a number of concerns, not drugs alone, that bring children into care. The adult behaviours related to meth and other complex issues, can involve violence and abuse to children."

She said they took the welfare of children affected by meth very seriously.

"Not only are they at risk of inhaling, absorbing and ingesting chemicals, there are common themes around families who have drug issues.

"They often have trouble affording food, and paying the bills, children from these families are often neglected, do worse at school, and suffer from increased health problems."

She said they are committed to making sure they are able to live in a safe and nurturing family environment and make an alternative care arrangement when necessary.