Detective Senior Sergeant Rhys Johnston, of the Mid North police Criminal Investigation Unit, said it appeared a small indoor cannabis growing operation had ''gone awry'' and started a fire.
''These operations do draw a lot of power and have high heat sources. It's always risky.''
A woman had been charged with cultivating cannabis and would appear in the Kaikohe District Court, he said.
Earlier, about 10.45am, the Kawakawa brigade was called to put out a fire in a 20m-high pine tree on Waikare Valley Rd, east of Kawakawa.
Deputy fire chief Alistair Leitch said it was a mystery how the fire, which was about halfway up the tree, started because there were no powerlines in the area and there had been no lightning strikes.
The core of the tree was burning so the volunteers used a chainsaw and a thermal imaging camera to try to reach the seat of the fire.
There were concerns the tree, which was already on a lean, could burn through and fall across the road. The brigade contacted the Far North District Council which dispatched a crew from Northland Treeworks to cut down the tree before it fell.