There has been a second explosion at the Pike River mine in New Zealand, leaving no hope of survival for the 29 men missing underground.

Police spokesman Gary Knowles has spoken to the media, saying the explosion was severe and far larger than the first blast on Friday.

"It was extremely severe," he said, adding that it occurred at 2:37pm local time.

"It is our belief that no-one has survived and that everyone has perished."

Relatives of the victims were given a briefing before police told the media of the second blast.

Many family members were seen in tears as they left the briefing.

The 29 men had been missing underground for almost six days.

They included Australians William Joynson, 49, and Joshua Ufer, 25, who are both from Queensland.

Pike River Mine chief executive Peter Whittall has ruled out the possibility that any of the rescue operations caused the latest explosion.

"I am absolutely confident the measures taken up there by the rescue teams and men up on site were correct and absolutely nothing done up there could have caused this," he said.

"Gas had been building up. It is a natural thing that could have happened on the second day or the third day. It just happened today."

Mr Whittall says he has a lot of respect for the families he has been working with.

"The question now is what next?" he said.

"I still want [the miners] back and their families want them back, but the mine is just as unsafe now as it was two hours ago."

Darkest hour

Grey District mayor Tony Kokshoorn was on top of the mine where authorities were checking gas levels when he says "the whole thing just blew its top".

An emotional Mr Kokshoorn says it is the area's darkest hour and that families are inconsolable.

"This is the West Coast's darkest hour. It doesn't get worse than this," he said.

"The reaction in there is just devastation. They just sobbed openly and fell to the floor.

"Many people were just shouting out in anger. They were just sickened by the whole thing."

Mr Kokshoorn says authorities knew straight away there was no hope for the men trapped following the second explosion.

"At that point we just started consoling each other," he said.

"All I know is our men are gone."

He says family members of the victims have directed some anger at authorities.

"There was some anger directed at the police. Certainly not at Peter Whittall."

Mr Whittall has now requested privacy for those who lost loved ones in the disaster.

Poisonous gases

Earlier today, high levels of toxic gases came up through a bore hole that was drilled into the mine.

Rescue crews had been repeatedly held off from entering the mine because of the presence of poisonous gases and the high risk of underground fires and explosions.

A robot had been sent down the mine earlier today and found a miner's helmet with its work light still on.

It is believed, however, that the helmet belonged to one of two men who managed to walk out of the mine following the blast on Friday.

Yesterday, authorities released photos of the 29 miners.

The rescue team also released CCTV footage of the initial explosion at the mine, which showed debris and dust being blown out of the mine's main entrance.

Authorities had also managed to get a camera down to an area where there were reserves of oxygen, however it showed no signs of life.