Northland firefighters are trying to contain a fire at an abandoned sawmill.

Kaikohe Fire Chief Bill Hutchinson says the fire is burning through a huge pile of sawdust.

He says to make matters worse, the sawdust covers a peat swamp, which means the fire is burning metres below the top of the pile.

This means even in areas where the sawdust's surface is not on fire, it could still be burning several metres below.

Mr Hutchinson is worried that one of the fire fighters will fall through the sawdust into a burning pit.

Diggers are now creating a trench to contain the fire.