The two men who managed to get out of the Pike River mine on the West Coast near Greymouth, after an explosion trapped 27 of their fellow workers, clambered up a narrow chimney to escape.

Emergency services are at the mine entrance this morning and hope to mount a rescue bid once fears about dangerous air quality are resolved.

The trapped men - 15 mine employees and 12 contractors - were this morning still unaccounted for following an explosion at the isolated, underground mine, 46km northeast of Greymouth, yesterday.

The incident was discovered about 3.50pm when an electrician went into the mine to investigate a power outage and discovered a loader driver had been blown off his machine about 1500m inside the mine shaft, police spokeswoman Barbara Dunn said yesterday.

Two men, Russell Smith, 50, and Daniel Rockhouse, 24, fled to the surface following the explosion, and were being treated for moderate injuries at Grey Base Hospital.

When they emerged, Mr Smith and Mr Rockhouse indicated three more workers were on their way out from the mine, but by this morning there was still no sign of those workers.

Ron Boddy, a Pike River miner who was off work yesterday told the Dominion Post mine colleagues last night told him that the two men had managed to clamber to safety up the air return - a narrow shaft that doubles as an escape route.

"It is pretty damn high [a few hundred feet] so it would have been hard going.

"They could all get out the same way but it would be one man after the other - that's how tight it is."

Greymouth mayor Tony Kokshoorn revealed that one of the missing miners is a local councillor, Milton Osborne.

Search and rescue workers gathered at the mine last night but were unable to enter amid fears about poisonous gas inside.

One concern was that the mine might still have pockets of "fire damp" or explosive methane gas, which could not be ventilated because the mine's fans were not working.

Despite the ventilation not working, Pike River said those in the mine should have access to fresh air.

The company said all miners were equipped with self-rescue devices, which generated oxygen from their own breath.

Tasman Police district commander Superintendent Gary Knowles last night travelled from his Nelson headquarters to oversee the police team who were gathering in Greymouth.

"This is a very uncertain and worrying time for families and friends of the miners and contractors who are at the mine," Superintendent Knowles said. "We are working closely with mine officials and other emergency services to do everything we can to help with the rescue operation."

Meanwhile, medical services are on standby to help with rescue efforts.

Six ambulances were at the mine's processing plant in Atarau, halfway between Greymouth and Reefton, and three rescue helicopters had been sent from Nelson, Greymouth and Christchurch.

Grey Base Hospital activated its emergency response plan yesterday afternoon.

The West Coast District Health Board said it was working with other emergency agencies and had support available from other District Health Boards and the Ministry of Health if required.

Family of the trapped miners yesterday mounted a vigil at the cordon on a road leading to the mine, but were later offered space at a welfare centre at the Red Cross Hall in Greymouth to await news from the site.

New Zealand mining expert Dave Feickert said while it was not known at this stage what had caused the blast, if methane gas had mixed with coal dust the explosion would have been very big.

"The biggest single problem is to find out where the guys are, and the communications systems always go down," he told NZPA.

"We're all trying to develop a communications system that would survive an explosion of this nature."

People who were right inside the mine in the production areas would be at the greatest risk, he said.