The Pike River Royal Commission of Inquiry will hear from former mine managers and Australian experts next month, when it finally asks 'what happened?'

Fifteen months after 29 men lost their lives in an underground explosion, the commission has so far spent weeks probing the search and rescue effort and the Pike River Coal Company's set up, and is now ready to look into what caused the fatal blast.

Some evidence has been temporarily suppressed, but early hints have emerged:

- Pike River logs books in the month before the disaster recorded soaring gas levels, and a leaking gas drainage line.


- Miners had been repeatedly caught taking contraband underground, including cigarette lighters, and some gas sensors had been tampered with.

- There were problems with the new underground fan.

- When Mines Rescue reviewed footage of the portal, they noticed a rag used to indicate airflow was behaving oddly in the minutes before the explosion.

- The commission itself has identified hydro mining as a 'prime suspect', although there may not have been an ignition source there. However, it was the gassiest part of the mine and there had been one roof collapse there.

One of the most intriguing pieces of information is a recording between the control room on the surface and the coal face 2.4km underground, in the lead up to the explosion. The water pumps to the coal-cutting machine were turned on only a minute or two before the blast.

Control room operator Dan Duggan initially made contact after turning the water on, when someone replied: "Hello Dan, who you looking for?" followed by an unidentified, muffled noise. All further contact was lost.

Mr Duggan's brother Chris was among the 29 dead.

The commission resumes on February 8 with evidence from Department of Labour services and support general manager Brett Murray, followed by Australian David Reece, who provided the department's investigative team with expert advice on mine design and ventilation.

In week two, another Australian mining expert, Tony Reczek, will discuss electrical issues.

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

Appearing late last year, expert witness Dr Kathleen Callaghan, from the University of Auckland, when describing the disaster said the cause was probably like Swiss cheese - all the holes lined up on that fateful November day.

The hearings are held at the Greymouth District Court and are open to the public.