The Government says it is checking whether leaked video footage from inside the Pike River mine was made available to investigators and whether it contradicts earlier assessments that the coal mine is unsafe to re-enter.

Families of the 29 miners killed in the mine are angry that they were never shown the footage, which was taken by a robot three months after a fatal explosion in November 2010 and appears to show two Mines Rescue workers inside the mine's drift.

The families say the footage, which was released to Newshub, backs their long-held view that manned re-entry of the mine's 2.3km drift was possible.

Police controlled the mine at the time the video was taken, and said the footage was not supplied to families of victims or the Royal Commission because it was assessed as having no evidential value.

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Acting Minister of Conservation for Pike River Issues Nick Smith said today he wanted clarification about the level of disclosure by police and whether its contents contradict safety assessments carried out by Solid Energy which concluded that manned re-entry was unsafe.

In particular, he wanted to know the location of the Mines Rescue workers shown in the video, telling Radio New Zealand it was "crucial to whether this video evidence is significant or not".

There is disagreement about whether the workers are at the mine's entrance or deeper into the drift.

Prime Minister Bill English said this morning he had been advised by police that the workers were in the portal at the entrance of the mine - where there is little or no methane in the air.

Former chief mines inspector Tony Forster, on the other hand, said the workers were around 300m to 400m into the drift: "There is no question about that."

"PAUSE FOR THOUGHT"

Forster said that meant the men would have had to cross over the seal between fresh air and "methane-rich" air.

He was struck by their "calmness" and "routineness" in the video, he told Radio New Zealand.

"And that probably comes across to a lot of people. There's no great haste and there's no panic, there's no crisis. It's a very calm, considered approach."

The footage also "vividly demonstrated" how accessible the mine was, he said. The footage shows the drift's roof and walls were not badly damaged by the gas explosions and that the robot was able to get 1600 metres into the mine.

"A lot of mythology has built up about the danger of entering the mine. And I think this is very clear imagery that normalises that for many people."

He said the footage would give the Government "pause for thought".

"The question would beg itself, if we knew the drift was absolutely secure up to that point, why not de-gas the drift up to that point, put a seal in ... and then if necessary go forward again."

NO NEW INFORMATION

Smith said it was not "secret or new" that people had re-entered the mine up to several hundred metres. This had been required to put a seal in place, he said.

He also said there was "no new information" about the good condition of the drift, which geotechnical reports had previously confirmed.

Labour Party leader Andrew Little, however, said the footage appeared to show that going into the drift did not pose the danger the Government and mine owner Solid Energy, a state-owned enterprise, claimed.

"The Government must immediately release all Pike footage and images and explain why it has kept the public and the families in the dark for so long," he said.

The video made a lie of Smith's claim that the atmosphere in the drift was highly flammable, he said.

In the video footage, smoke appears to be seen coming from the robot when it is deep in the mine's drift.

Smith said some had "over-interpreted" the video by suggesting that the robot would have exploded if the mine was filled with methane.

Risk assessments had clearly showed both oxygen and an ignition source were needed to cause an explosion, he said.

This was backed by Forster, who said that even if the robot ignited it would not have caused an explosion because the mine's atmosphere was 98 per cent methane, or "totally inert".

NOTHING HAS CHANGED - ENGLISH

The footage was first offered to the families by a source a month ago, and some of the family members travelled to Wellington to view it last week.

English said this morning that he was unaware of the footage until last night, when it was broadcast on the 6pm news.

He told Newstalk ZB's Mike Hosking this morning the footage changed nothing, as any decision to go into the mine had to be made under work and safety regulations.

Health and safety laws would have to be amended to allow re-entry of the mine so that directors were exempted from any liability. Labour has proposed such a move, but the Government has ruled it out and the Solid Energy chairman Andy Coupe has said he would rather resign than get an exemption.

Re-entry would also require a second exit to be built to the mine, which the company says would cost an estimated $100m.

After an agreement in February, the Government has been working with the families on unmanned re-entry of the mine, possibly by using another robot or a drone.

However, talks over that plan have become strained because the families wanted it to leave open the possibility of manned re-entry.

They were also upset that police had not released historic footage taken inside the mine.