The Labour Department had expressed concern about the stability of a roof in the Pike River mine, which experts believe collapsed and caused the initial explosion in the mine.
The Pike River Royal Commission today heard for the first time heard what experts believe caused the November 19 blast in the mine.
A Labour Department investigation found the most likely cause of the initial explosion was a collapse in the "goaf" of the mine, which is cavity deep within the coal extraction area.
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This collapse would have released methane gas, which ignited when water pumps in the mine were turned on by Daniel Duggan in the control room above ground.
The commissioners have made it clear that Duggan, who lost his brother Chris in the mine, did nothing wrong.
Under questioning from the families' lawyer Nick Davidson QC, the national services and support general manager for the Labour Department, Brett Murray, said a report was sent to Pike River mine just 25 days before the explosion expressing concern about the stability of that roof after excavation work to increase the size of the cavity by 15 metres.
An expert adviser on mine design and ventilation, David Reece, told the Royal Commission there are still many contributing factors that cannot be ruled out, including whether the electrical plant may have caused sparks deep in the mine, triggering the explosion.
However, Peter Whittall's lawyer Stacey Shortall has disputed that finding, saying contraband like cigarettes or matches in the mine could have been the cause, despite warning signs and random searches.
That's upset spokesman for the victims' families Bernie Monk. "She's got people to protect and she's trying to put the blame on our guys underground who cannot answer them," he said.
The family today demanded that a camera be sent down to the goaf area to give a clearer picture of what happened.
The Royal Commission resumed hearings in the Greymouth District Court today after a two-month break and an Australian expert on mine ventilation is due to take the stand.
Brett Murray of the Labour Department said the investigation is the largest of its type that has ever been undertaken by the department.
This third phase of the inquiry is scheduled to continue until Friday next week.
Meanwhile, the Royal Commission's request for a six-month extension has been granted.
Its final report will now be made to the Government at the end of September.
The report was due back by the end of this month.
But the commissioners said their timelines and management of evidence and hearings were being complicated by the parallel inquiries being conducted by the police and the Department of Labour.
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