Exclusive: Did a conveyor belt cause the deadly Pike River blast?

Investigations are underway into whether actions by police or Pike River Mine management caused the second explosion that ended all hope of finding any of the men inside alive.

At issue is whether the mine's conveyor belt was turned on during the police-led rescue operation, and whether it could have triggered the second blast.

Rarely seen footage taken from down at the portal shows the second explosion, which is now of significance.

If there were any survivors from the first blast, all hope of saving them ended then and police called off the rescue.

The question is: did someone turn on the mine's conveyor belt, creating a spark which ignited the methane in the mine?

A 'clunk' sound can be heard in the footage, which some say is the conveyor belt starting up. The belt can be seen starting to move before the mine explodes.

Dean Dunbar's son Joseph was one of the 29 men killed, and he's spent the last two years investigating the conveyor belt.

The belt was originally used to transport coal or excavated rocks out.

Official documents show authorities considered turning it on to check if the mine was obstructed - or to give survivors a chance to ride it out.

"I guess they had to try something," Mr Dunbar says. "They had to try to give these guys a chance."

He was leaked a risk assessment which showed police and Pike River management were discussing it as an option, deeming it a "moderate risk".

"All the documents said that this could cause a secondary explosion," Mr Dunbar explains.

Pike River bosses Peter Whittall, Doug White and Steve Ellis were closely involved in the rescue operation, but police had control of the mine.

Gary Knowles was the incident controller, and was on site at the time.

The operation commander was assistant commissioner Grant Nicholls, based at New Zealand Police national headquarters in Wellington.

A document shows Grant Nicholls prepared a strategic decision document about the operation of the conveyor belt before the explosion.

Police won't release that document.

"There's something not right here," Mr Dunbar says. "What the hell is going on?"

But police have started investigating: Detective Superintendent Peter Read told Mr Dunbar he has begun an investigation plan "that will look at whether the conveyor belt was started on the 24th November 2010 and determine who are the right people to speak to".

Police told Newshub on Tuesday afternoon that based on their current knowledge, they have no information to suggest the belt was switched on after the first explosion.

They say mine experts have viewed the footage and concluded the movement of the belt was actually caused by the second explosion.

They also say a warning siren would have sounded if the belt was activated, but they have no report of that happening.

But police have acknowledged they did a risk assessment, and they say the officers they have spoken to so far say it wasn't approved and the belt was not started.

Police say they've taken the decision to investigate as the most efficient way to answer questions for the grieving families.

Mr Dunbar says finding out the cause of the second explosion is crucial.

"That caused the fires underground. That caused the devastation at the end of the drift and that possibly killed the survivors if there were any. This sealed the fate in so many different ways."

Extensive efforts were made to contact mine management, but they either could not be reached or did not want to comment.

It's been eight years since the Pike River tragedy, and what really happened is still being uncovered.