Police believe there is no chance of any of the trapped 29 miners surviving the second massive explosion in New Zealand South Island's Pike River Coal mine on Wednesday.

Police said the explosion, at 2.37 p.m. on Wednesday, was so powerful that no one could have survived it.

The blast was at least as strong as the first one, they said.

Police say there is now no chance of the miners being alive and they are going into recovery mode.

The 29 men have been missing since the first explosion last Friday.

Grey District Mayor Tony Kokshoorn, who was at the civic center when the news was broken to the miners' families, said "this is the West Coast's darkest hour".

He said the news had been "sickening", and families were devastated.

Emerging from the briefing, rescue team chief Gary Knowles said there was an extremely severe explosion this afternoon and police believed there could be no survivors.

"We are now in recovery mode," he said.

He said it was his most tragic moment as a policeman.