FAMILIES of the 29 Pike River mine victims - including two Australians - who perished in today's second blast fell to the floor screaming and were in "absolute despair", when they heard the news.

Late this afternoon police confirmed there was an explosion at the West Coast mine and they believe there is no chance any of the miners survived.

Superintendent Gary Knowles said the miners couldn't have survived a blast of that magnitude, which occurred at 2.37pm.

Prime Minister John Keys later said flags would fly at half-mast around New Zealand on Thursday to mark the deaths of the miners.

Family members were seen crying as they left a police briefing after the explosion.

"Unfortunately I have to inform the public of New Zealand at 2.37pm today there was another massive explosion underground and based on that explosion no one would have survived," Mr Knowles said.

"We are now going into recovery mode. I had to break the news to the family and they were extremely distraught.

"I was at the mine myself when it actually occurred and the blast was prolific, just as severe as the first blast. Indirectly based on the information no one would have survived."

Family members were emerging from the meeting crying and some were shouting at media and police. There are a large number of police at the scene.

Miners wouldn't have suffered

Mine safety expert David Feickert told TVNZ the miners would have not felt the explosion.

"Chances are they became unconscious from the carbon monoxide and wouldn't be affected by the explosion," Mr Fieckert said.

"From the samples today [we know] that there was some kind of heating going on, that provides an ignition force."

He said that given the lack of air ventilation methane gas would have built up a combination between it and the smoldering material would have caused the explosion.

Toxic gas erupts from drill shaft

HOPES of finding the 29 trapped Pike River miners - including Queensland pair Josh Ufer and William Joynson - alive are beginning to fade despite a defence force robot finding a miner's hat with the light still on.

A drilling team which broke a narrow shaft through to the section of the New Zealand coal mine was hit by a blast of potentially deadly gases from inside.

And officials have become increasingly pessimistic about the chances of pulling the men alive from a network of tunnels some two kilometres deep in the side of the mountain.

Toxic and potentially explosive gases have kept rescuers from entering the mine, though an army bomb disposal robot crawled one kilometre into the tunnel on Wednesday and found a miner's helmet with its fixed light still glowing.

Drillers using a diamond-tipped drill bit to prevent sparks finished boring a 160-metre hole to the mine's main tunnel, close to where the missing men are believed to have been at the time of the blast.

It was a key step, giving officials their first information from that section of the mine and allowing testing for levels of dangerous gases.

Hot air and gas rushed the hole when the chamber roof was punctured Pike River Coal chief Peter Whittall said.

"As we expected, but not as families hoped, the air that came out from the hole was extremely high with carbon monoxide, very high in methane and fairly low in oxygen," Whittall said.

"It's making their hopes diminish and making it more difficult for them to hold out that hope that all 29 of those guys are waiting for us as we hoped on day one.

"I think we're all coming to that realisation now," he added, but he would not comment on whether the gas levels were survivable.

For the first time police also conceded they may never reach the miners.

"The samples are off the limit," said police superintendent Gary Knowles, who delivered the grim news to the families before addressing a press conference on Wednesday morning.

When asked whether it would ever be safe for rescue workers to enter the mine, Knowles said "it may never reach that point".

"Whilst I can understand the frustration (of the families), we are doing everything possible to go underground. Obviously over time, hopes diminish."

Carbon monoxide is extremely poisonous, while explosive methane is the gas believed to have ignited in Friday's blast.

"The environment is still unstable, it is unsafe and it is not appropriate to send rescue teams underground at this time," Knowles said.

Whittall said the helmet that was spotted by the robot belonged to Russell Smith, one of two miners who were not as deep inside as the missing workers and who stumbled to the surface with minor injuries shortly after the blast.

He said it offered a small hope that if any of the missing miners had survived, they may not be in complete darkness.

Rescuers have two robots at the mine, and were scheduled on Wednesday to take delivery of a third robot, which they hope will give a clearer picture of conditions underground.

Only the third robot, which has been sent from Western Australia, has the capacity to explore deep enough into the mine to potentially spot the miners, officials said.

Rescuers also plan to feed a camera down the bore hole.

Prime Minister John Key has warned the nation to prepare for the worst, as frustration grew among some relatives of those missing.

"We've got to know, we've got to go and have a look," Geoff Valli, whose brother Keith, 62, is missing, told National Radio.

"Around town a lot of guys are prepared to go in and do it. It's time for men to do what men have got to do."

Knowles said he can understand the mounting pain of the families, but safety for rescuers is paramount and the dangers - including that of another explosion - were too great.

Authorities on Tuesday released chilling video footage which showed a powerful, sustained explosion at the mine on Friday.

The video showed stone dust being blasted for 52 seconds out of the shaft's entrance - some 2.5 kilometres from the explosion and where the missing men were located.

New hope as NZ miner's hat found

A DEFENCE Force robot sent into the Pike River coal mine has found a miner's hat with the light still on, media were told this morning.

The robot broke down in the mine yesterday but was restarted and sent deeper into the mine - stopping at 1km, with a second Army robot also moving into the tunnel.

Pike River CEO Peter Whittall said the discovery of the hat belonging to miner Russell Smith - who escaped the blast - could be good news as it meant those trapped could still have light after four days.

"If they've been using their lights sparingly may have light with them through all this time," he said.

"I certainly hope the guys are waiting down there, I certainly hope to see them again.".

The robot had picked up other debris but nothing of a material nature, he said. The second robot was 800m into the tunnel.

Some footage had been obtained from the robots and would be shown to miners' families at 4pm, a police spokesperson said.

Australian Robot

A more advanced robot from the Perth Water Board in Western Australia has also arrived on the West Coast to help in the rescue.

Tasman police district commander Gary Knowles told this morning's media conference the Australian robot would be shipped to the mine site today.

He said it was "a very large beast" and rescuers would be assessing its capability and then deploying it.

Mr Whittall said the robot would need to get past the loader that was in the middle of the underground roadway.

Air New Zealand is shipping in another robot from the United States at 7.15am on Thursday.

Mr Whittall reiterated that the issue of rescuers entering the mine was not about them being unable to breath, but the risk of an explosion happening independently of their actions.

"If an explosion happens while [a robot] is in there we've lost a piece of equipment - not a person."

'Time to go in'

Geoff Valli, whose brother Keith is down the mine, earlier told Radio New Zealand he was tired of hearing about the delays in the rescue.

"They need to go in and have a look," he said. "We ain't got the time."

Mr Valli acknowledged he would feel "absolutely terrible" if there was a second explosion and rescuers lost their lives, but questioned how the rescuers will feel if the miners lost their lives and could have been saved.

"It's time for men to do what men have got to do," he said.

Mr Valli said his brother had been mining all his life and was a "good bugger".

"He's quiet, unassuming, as solid as a rock."

Mr Valli said he had hope, but that it was "not there for ever".

No evidence of survivors at fresh air base

Footage taken from a camera lowered into a fresh air base, to which miners could retreat if necessary, did not reveal any evidence of survivors, Mr Whittall said.

He said the footage showed no-one had visited the base since the explosion.

The footage was very poor quality, he said

"There's not a lot of light - it's pretty much just black. It doesn't show anyone's there, doesn't show anyone was there."

He was critical of reports rescuers could have safely gone into the mine if they had entered soon after the explosion.

Australian mining expert John Brady earlier told the Herald there is a small window of opportunity after any mining explosion where methane levels have not yet built up.

But Mr Whittall said it would always have been dangerous to enter the Pike River mine.

"It would have been just as hazardous straight away as it is now."

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