The man who led the Department of Labour's investigation into the Pike River disaster says the inability to get into the mine to examine it posed real problems for his team.

Brett Murray is currently giving evidence at the third phase of the Pike River inquiry, which continues today after a two month break.

He told the Commission the investigation was the biggest the department had ever undertaken, and came with several challenges.

"There was the lack of access to the mine itself and that meant that the investigation had to be very broad in its scope in order to reconstruct a detailed picture," he says.

"Inability to do a scene examination also resulted in difficulty in establishing with direct causation of the explosion."

Mr Murray says the disaster may have been prevented if a tube bundling gas analyser was in place.

The lack of a tube bundling system in the mine has been criticized throughout the inquiry.

Under cross-examination from families' lawyer Nicholas Davidson QC, Mr Murray said things may have been different if it was installed.

"Tube bundling system would have formed part of their methane management system so in addition to the real time monitoring, it would've allowed greater analysis of gas trending. Obviously it would have obvious benefits after the fact of the explosion," he says.

The inquiry has also heard that the disaster was probably caused by a large collapse in the mine face, which would have sent methane rushing through the mine.

Today's resumed Royal Commission has heard that's the preferred theory as to what happened.

Justice Pankhurst stressed however this was based only on the most likely probabilities.

About the same time, the surface control room operator had turned on the water pumps back into the mine which in turn re-powered the underground electrical system - which may have sparked.

The Commission will also hear from Australian David Reece, who provided the Department's investigative team with expert advice on mine design and ventilation and next week another Australian mining expert will discuss electrical issues.

Former Pike River Mine manager Doug White and technical services manger Pieter van Rooyan will also appear.

A report was due back by March however the Commission says its timeline and management of evidence and hearings would complicate it by the parallel inquiries conducted by the police and the Department of Labour.

Photo: Brett Murray is currently giving evidence (Deidre Mussen)