Solid Energy estimates that unmanned re-entry of Pike River is at least three or four months away.

Chief executive Tony King said that the robot selected as the preferred option for re-entry was based on a stock item but with protective coverings removed to allow it to fit down a 150mm borehole.

"It will need to be built to a modified state, with all aluminium removed and replaced with other metals," he said.

"If technically feasible a real-time gas monitor would be installed so that gas conditions in the mine could be monitored by the robot.

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"It is estimated that completing the necessary robot modification and planning and preparation of boreholes will require at least three to four months once the re-entry plan is signed off."

Commenting on video footage shot from a borehole at the far end of the mine King says that video footage from inside the mine does not in any way contradict the company's decision that manned entry is not safe.

The project team working on the unmanned exploration had identified a preferred option capable of going down a borehole, unfolding and then moving around.

"This follows two major meetings with interested parties, including relevant government agencies, technical experts and an advisor to the Pike families, to scope out options and develop the exploration plan."

The process was necessarily time-consuming, he said.

"There are already four abandoned robots in the mine - we need to ensure that any future unmanned exploration is conducted with the right technology to get the job done effectively."

The option currently being considered was to drill a new borehole in an unexplored part the drift to maximise the area of unexplored drift that the robot could cover.

Up to 200 m of unexplored drift could be covered with one borehole, which would be a major step forward, he said.

Anna Osborne, left, and Sonya Rockhouse during a vigil to mark the second reading of the Health and Safety Reform Bill in 2015. Photo/ Mark Mitchell
Anna Osborne, left, and Sonya Rockhouse during a vigil to mark the second reading of the Health and Safety Reform Bill in 2015. Photo/ Mark Mitchell

Meanwhile Prime Minister Bill English today dismissed as "nonsense" any suggestion there had been a cover-up over the video which showed a wooden pallet apparently unaffected by fire.

The footage, released to the families of the 29 men who died in the mining disaster, was aired on Newshub on Sunday.

It was shot in 2011 by a camera lowered down borehole 44, which was drilled after the explosions.

The families say that contradicts the story they were told, that everything in the mine had been burnt up.

The grainy footage also appears to show a pair of spectacles which may have belonged to Ben Rockhouse, one of the victims of the explosion who was working in that location.

His mother Sonya Rockhouse said the footage raises serious question about the Government's refusal to allow recovery specialists to go inside the mine.

"New Zealand has been told there's nothing but ashes and dust down there, they've been told that the drift can't be made safe to enter and investigate.

"That's just not true, my boy is down there and if pallets and paper are intact then so is his body."

English said it was known that there was no reason to believe that the same thing happened in every part of the mine but there was no denying there were 29 people dead and there had been a very large explosion.

Labour leader Andrew Little said there was mounting evidence to say "there's something in there we haven't been told or doesn't match up with the official story."

New Zealand First leader Winston Peters said people were not being the truth about Pike River.

English: "I think the implication that somehow there has been a cover-up is complete nonsense. Go and read the Royal Commission report about what happened."

He said the issue of re-entry should not be a subjet for discussion in any post-election negotiations.

Meanwhile, the Supreme Court has granted leave for some family of miners to appeal a decision by Worksafe NZ to drop charges against former Pike River chief executive Peter Whittal in relation to the deaths.

The case has been taken by Sonya Rockhouse and Anna Osborne, who lost her husband, Milton, in the disaster.