The investigation by regulator EnergySafety found clashing high voltage wires sparked the January 16 blaze, which ripped through a pine plantation atTwo Rocks.
Hot metal globules from two 22 kilovolt conductors on two adjacent poles fell on to dry grass and stubble.
EnergySafety director Ken Bowron said a bolt securing a conductor on one of the poles had been split, reducing the tension in the conductor and thus reducing the distance between it and another conductor, causing them to clash.
“The clashing of conductors… raises questions aboutWestern Power’s inspection and maintenance system for its power poles,” he said.
“Had the defect been identified and rectified, the conductor clearances would have been maintained and clashing would have been much less probable.”
The fire had occurred on a 33 degree day, with low humidity and winds gusting to 50 kmh, “precisely the conditions” likely to generate a blaze, Mr Bowron said.
EnergySafety andWestern Power would work to assess whether the fire was an isolated incident or whether it was “an indication of a more systemic problem”, and to see if further action was needed, he said.