A mining expert has told the Pike River Royal Commission that systems for checking levels of methane in the mine were faulty at the time of the disaster.
The hearings are now looking at the immediate causes of the explosion which killed 29 men.
A Department of Labour investigation has found the most likely cause was a sudden release of methane from a roof collapse in one part of the mine which was ignited by sparks from an electrical fault.
The Australian expert who provided advice to the investigative team, David Reece, said two methane monitors in the same part of the shaft at the time were producing different readings.
