[The Sunday Morning Herald] A second explosion has rocked the NZ mine where 29 men are trapped and authorities have told their families that none of them would have survived.

The man in charge of the rescue operation, Superintendent Gary Knowles, said that he was at the mine when the blast occurred, and it was horrific.

"Unfortunately I have to inform the public of New Zealand that at 2.37pm [12.37pm AEDT] today there was another massive explosion underground and, based on that explosion, no one survived," Superintendent Knowles said.

Superintendent Knowles has broken the news to the families, who were "distraught".

"It is our belief that no one has survived," Superintendent Knowles said.

He said the men had "perished".

"We are now going into recovery mode."

Distraught family members left a scheduled briefing at Grey District Council a short time ago, only about 10 minutes after it had started and many of them in tears.

Lawrie Drew, the father of trapped miner Zen Drew, said officials had told the families that there had been a second explosion.

Superintendent Knowles said it was one of the most horrific things he has had to deal with as a policeman.

"We had to break the news to the family and they are extremely distraught.

"I was at the mine myself when the explosion occurred. The blast was horrific. Just as severe as the first blast - and we are now moving into a recovery mode."

"I was at the mine myself when this actually occurred and the blast was horrific, just as severe as the first blast."

Superintendent Knowles would not speculate on what caused the blast nor would he say whether anyone else had been injured.

Grieving families left the afternoon briefing and did not want to talk to the waiting media.

An ambulance arrived to offer assistance to the family members.

A formal media briefing will be given at 5.30pm (3.30pm AEDT) at Greymouth police station.

Grey District mayor Tony Kokshoorn broke down as he left the briefing.

"We thought we were going to get some good news," he said outside.

"They told us there was a massive explosion and there was no way there were any survivors."

He said family members dropped to the floor after hearing the news, with many angry at how police had handled the operation.

He said the irony was that police were now sending in a team into the mine.

"It's just gut-wrenching. This is the west coast's darkest hour," he said. "It doesn't get any worse than this."

He said the second explosion was "far bigger than the first".

"What we've got is a very angry group of families."

He said it appeared a lethal mixture of gasses had ignited the mine.

Earlier today

Earlier today, Police Commissioner Howard Broad had issued his most bleak outlook yet on the trapped miners.

Hot air and gas had rushed up a bore shaft from the mine as robots captured new footage, including a miner's helmet with a light still on.

A fire continued to burn inside the mine, and abandoned mines on the West Coast can burn decades after closing to mining.

Commissioner Broad had suggested it could take "quite a long time" before the environment down the mine was safe to enter.

"And that, of course, makes the outlook for those who were down in the mine a very bleak one," he said.

Anger growing

Before the second explosion this afternoon, the brother of the oldest miner trapped at Pike River had said it was time to send a rescue team into the mine, no matter the risks.

Geoff Valli, brother of Keith Valli, 62, told Radio New Zealand this morning he'd had a "gutsful" of hearing the same excuses from police day after day. Police had been warning of the volatile environment in the mine.

Speaking quietly and with his voice breaking, he said he did not feel they could wait any longer.