Deputy fire chief Karen Fry says the structure at the Mackenzie mill, along the Fraser River, was in the process of being demolished.
She says the owners of the mill were trying to get the mill operational since the last fire in 2011, following another one several months earlier in 2010.
Fry says the area that was being renovated was not affected by the blaze and that a millwright at the scene was hopeful the mill could still be opened on schedule.
The cause of the fire has not been determined but Fry says the building's heavy timber frames would have burned for a long time, allowing the blaze to spread.
Fry says a preliminary investigation indicates that there was no suspicious activity.