Prime Minister John Key is devastated by the tragedy at the Pike River coal mine and is sending his condolences to the families of the men who lost their lives.

A second explosion in the mine this afternoon - significantly larger than the initial blast - snuffed out all remaining hope of any survivors among the 29 miners who had been stuck in the mine since last Friday.

Addressing media this evening, Mr Key said it was a national tragedy.

"The 29 men whose names and faces we have all come to know, will never walk amongst us again. This is a national tragedy.

"To lose this many brothers at once strikes an agonising blow. Today all New Zealanders grieve for these men. We are a nation in mourning.

"After days of waiting, of both preparing for the worst and hoping for the best, they have been delivered the cruelest of news.

"To all those who have lost a loved one, New Zealand stands shoulder to shoulder with you. Though we can not possibly feel this pain as you do, we have you in our hearts and our thoughts.

"We have you in our hearts and our thoughts you we had all longed for that miracle to occur, that your men would be return to you."

Families tonight lost husbands, fathers and siblings, he said.

"This is tragedy for the people of Greymouth. The loss will be felt in every home."

He said a Commission of Inquiry will be confirmed after Cabinet meets on Monday to investigate the cause of the explosion, and all other relevant factors.

A motion in Parliament tomorrow will acknowledge the tragedy, and the House will rise as a mark of respect.

Flags will fly at half mast on all Government buildings.

Mr Key will travel to Greymouth tomorrow.

He said he wanted to go and thank the people who had worked on the operation, and meet the families of those who had died.

He said many inquiries could take place.

There had been a lot of strain of emergency service workers, given recent events including the September earthquake that rocked Canterbury.

He said the Government will support families and possibly help out with financially, but those discussions had yet to take place.

"It is unquestionably a huge loss and an enormous tragedy, and I think that's reflected in the way the country is responding.

"From the moment we heard the news, we all recognised it was a very challenging environment."

He said he had met family members of the miners on his two trips to Greymouth.

"Looking in their eyes, you can see their pain that they just wanted their loved ones back."

He expected that rescuers still would not enter the mine until it was safe.

The incident had already claimed 29 lives, and no more lives should be lost, he said.

He understood the anger that some families had shown about the decision not to enter the mine.

But he said the reason was clear, that is was unsafe to enter the mine, and he was entirely confident that the operation had been run properly.

"The range of human emotions is natural and anger is clearly one of those."

"The volatility of that environment and the sheer risk it presented to the rescuers, prevented us from going there.

"It's nothing to do with bravery.... but putting rescuers in the mine without the appropriate sign that it was safe to do so would have been an unacceptable risk."