Breakthrough as rescuers drill in to mine's main tunnel

• Missing: Pete Rodger and Malcolm Campbell

When the drill hole – reaching 530ft into the mine – broke through, a jet of hot air and gases was released, Pike River Coal Ltd chairman John Dow said.

Experts immediately began testing samples from the bore hole, located near where some miners were working when the blast happened on Friday. There is now a plan locate a video camera down the hole to search for signs of the missing miners.

Speaking following the breakthrough, the Pike River mine chief executive Peter Whittall said initial analysis indicated that it was still too dangerous for rescuers to go in.

"The air from the (bore] hole was extremely high in carbon dioxide, very high in methane and fairly low in oxygen," he said.

Reporting on what he described as "another sobering meeting" with the families of the trapped miners, Mr Whittall said they had expressed frustration with the situation but also understood why nothing could be done yet.

He admitted that their hopes of finding all 29 men alive were "diminishing", but added that he hoped "the guys are were waiting there" and that he hoped he "will see them again."

Pete Rodger, 40, from Perthshire, and Malcolm Campbell, 25, from St Andrews, Fife, are among the miners missing following the explosion.

Rescue controller police superintendent Gary Knowles, said the air samples from the bore were "off the limit (and) the environment is currently unstable" in the mine."

"It is not possible to send rescuers underground today," he added.

In addition to the toxic gasses swirling in the tunnels, rescuers say that there is heat underground, believed to be coming from a smouldering fire.

Mr Knowles said that following one setbacka robot had been placed into the mine, following a second attempt, and had travelled a kilometre into the shaft where the helmet of one of the miners who escaped in the initial stages of the explosion was discovered, with its lamp still operating.

A second robot was to be sent deeper into the mine during the day, and video footage from both machines would be analysed.

A third, larger robot had been flown in from Australia and was being prepared to be sent even deeper into the mine.

Acknowledging the growing public frustration being expressed at the lack of action in the rescue attempt, Mr Knowles insisted rescuers were ready to go the moment it became safe to do so and were "doing everything possible" to speed the rescue effort.

He added they were prepared for all eventualities when the miners were finally reached.

His comments came as it emerged New Zealand's prime minister John Key had warned the families of the trapped men to prepare for the worst.

Mr Key said police were now planning for the possible loss of life following the blast in the mine.

He told the New Zealand parliament it was still too dangerous to enter the mine to find what had happened to the men.

The premier said he shared the families' frustration that a rescue team could not be sent in because of the toxic gas levels in the mine.

And he added that the miners were "tough, resourceful and stoic men" who looked after each other in the same way as a father looked out for a son.

Newly released security footage has shown a wall of white dust surging from the mine entrance and small stones rolling past for about 50 seconds as the force of the blast ripped out of the mine.

The dust was blown across a valley and the shock wave shot up a ventilation shaft, tearing off surface vents hundreds of feet above the blast.