Police Commissioner Howard Broad has admitted the situation for 29 missing miners at Pike River Coal is becoming "bleaker by the hour, by the day".

Mr Broad made the comments minutes after dramatic security camera film revealed the scale of the blast at the mine on Friday afternoon.

Officials have known about the film for days, but did not reveal its existence until yesterday.

It was shown to the families of the lost miners before being made public.

The film shows a protracted explosion in which dust was blown out of the mine's entrance for 52 seconds.

The screening marked a distinct shift in the mood of those involved in trying to reach the missing miners.

"As the situation continues, the situation for those who are below ground is bleak and gets bleaker by the hour, by the day," Mr Broad said.

The man running the operation, police superintendent Gary Knowles, described the situation as "bleak and grave".

"We have to understand the risk of a second explosion is real."

Mr Knowles said there was no chance of anyone going into the mine until police could guarantee their safety.

There has been no communication with the 16 miners and 13 contractors since the explosion ripped through the mine, 46km from Greymouth.

Pike River Coal chief executive Peter Whittall said he held some hope.

"What sustains my hope is the information we had from day one - survival or otherwise of a blast of that nature depends on where you are and what structure you are behind or where you are standing," he said.

"The other thing that sustains my hope is that we still have a compressed air line going underground. We don't know where it goes to."

But he conceded: "I think it is becoming obvious there may be not 29 guys all sitting together waiting to be rescued. How many of them are I don't know but those are the ones I hope are rescued and those are the ones I am waiting to see."

Mr Whittall said a camera had been lowered into one of the mine's fresh air bases, to which miners could retreat to if needed, but it had not shown any signs of life.

"The camera showed some minor damage to the area, signs that it had been knocked about and some general signs of disrepair.

"There was no one there, there was no sign of any person in the area ... no evidence that anyone had come into that fresh air base at any stage since the explosion."

The emotional press conference came after a day in which a Defence Force robot sent into the mine opening broke down little more than 500m into the shaft.

That angered some families who had placed hope in the robot travelling the mine's nearly horizontal 2.5km shaft to give them some answers.

Mr Whittall was questioned at the press conference about the decision to wait until yesterday to show the families of the miners the video film.

He said the reason it had been shown to families was because they had kept asking why, if there was fresh air in the tunnel, rescuers didn't go in.

The video, he said, showed the blast was large and violent and went on for a long time.

The tunnel entrance was the furthest point from the blast - at least 2.2km away and possibly more.

Questioned about withholding information from the families and the possibility of raising false hopes, Mr Whittall said he would not speak as to whether the film was withheld or not.

The families had always known there had been an explosion and that the dust from it had gone across the valley.

"That piece of information [the video] just hasn't been relevant to doing a rescue operation"

Family members had found the video sobering and had not asked as many questions as in their usual briefings.

They had been glad to see it, but it did not change anything for anyone.

"What it's done is graphically reinforce what we have been saying all along," he said."

Though Mr Whittall said families had been shown the film to show them how significant the blast had been, its sudden release coincided with the Herald questioning Prime Minister John Key about it earlier in the day.

Within hours, the video was shown to the families, then made public.

Gerry Morris, a former mining journalist who is from a West Coast mining family, said the families should have seen the film on Sunday.

"The families could have made their own call on the seriousness of the tragedy. They are quite capable of that, being mining families."