Phase three of the inquiry into the Pike River disaster resumed in Greymouth yesterday to try and establish exactly what caused the explosion at Pike River.

The New Zealand Department of Labour undertook a nine-month investigation into the most likely causes of the explosion.

Brett Murray, who headed DOL’s investigation into the tragedy, told the commission a significant roof collapse in the goaf of the mine had probably occurred, causing an accumulation of methane.

The goaf fall would have knocked over a stopping, resulting in high levels of methane diluting through the mine.

The explosive gas would then have had to come into contact with an ignition source, such as electrical arcing at the main fan, electrical arcing at the gas sensor, faulty machinery or “contraband”, resulting in the deadly 52 second explosion.

Murray said the ignition source would likely have been electrical.

DOL’s rigorous investigation into the disaster was completed following the input of five experts, providing advice and analysis on geology and geotechnical engineering, ventilation engineering, electrical engineering, chemistry and gas analysis.

The core group of experts included Safety Managers consultant David Reece, University of Queensland’s Minerals Industry Safety and Health Centre director David Cliff, expert in ventilation engineering Tim Harvey, ARA Risk Consultants senior consultant Tony Reczek and geology expert David Bell.

Murray said the Pike River investigation had been the largest investigation the DOL had ever undertaken.

Under cross examination from QC Nicholas Davidson, Murray confirmed the explosion was of a methane gas nature as opposed to a coal dust explosion.

The methane would have had to mix with the air at an explosive range of 5-15% then come into contact with an ignition source, triggering an explosion, Murray said.

While the DOL reached a likely conclusion that the explosion occurred because accumulated methane in the goaf was expelled by a large roof fall, Murray admitted the cause of the explosion may never be determined because it was almost impossible to determine the exact cause of the ignition.

A lot of holes remain in DOL’s investigation because of the inability for the team to venture into the mine to inspect the scene.

DOL’s ability to undertake an impartial investigation into the causes of the Pike River explosion was questioned by lawyer Nigel Hampton.

“Did your investigation include any scrutiny of the department itself in relation to what might have been seen as its possible contribution to what took place in terms of planning, development and actual mining within the Pike River mine?” Hampton asked.

Murray said the department had commissioned an independent report looking into its role, but admitted the department itself had not examined the role of the inspector.

The DOL has come under heavy scrutiny at the inquiry thus far, with claims it had not provided its inspectors who were overseeing Pike River with any adequate training.

DOL’s investigation led to Peter Whittall, Pike River Coal and VLI Drilling being charged.

Phase three of the inquiry into the Pike River disaster will wrap up on February 17.

The reporting date for the Royal Commission on the Pike River mine tragedy has been extended to September 28 to allow more time for the gathering of information.