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A rescue team preparing to search for 27 men trapped inside a collapsed West Coast mine are still assessing the safety of conditions inside the mine.
The rescue team was trying to make sure the vents were clear as there was a potential for a buildup of gas, police spokeswoman Barbara Dunn said.
Until that happened, it was unsafe for rescuers to enter the mine.
There was also a concern that ventilation inside the mine shaft may not be working properly without power.
"They're itching to get in there and start looking for other people and a bit frustrated at having to stand and wait," Ms Dunn said.
Families are gathering outside the site of the underground mine explosion, waiting for news.
The 27 men consist of 15 local employees and 12 contractors.
Two people have now emerged from the mine, following the explosion. They were flown by rescue helicopter to Grey Base Hospital with moderate injuries.
The two miners indicated three of their colleagues were also on their way to the surface, but there has been no contact with those miners.
CEO hopeful miners are safe
Pike River chief executive Peter Whittall said there were so far no confirmed deaths.
He described the safety conditions in the mine as excellent and said the trapped miners had safety equipment.
"Every worker carried a safety rescue device at all times including a breathing apparatus with oxygen," Mr Whittall said.
"The men are between two and two and a half kilometres inside, but because the mine drills into the side of the mountain they are probably only 120 below the surface.
"The issue for the rescue team will be to ensure that the ventilation underground is adequate for them to go in and find the men."
He said it was only speculation that the men were trapped because they were trained to go to a place of safety and to wait.
"I personally know every employee of the company ... I know what the shift is and who the men are on that shift, and I'm still waiting to get a full list of the employees involved," he said.
John Key: Details 'scant'
Prime Minister John Key understood there was a large explosion, but details of the size and scale were "scant".
"It has the potential to be a very serious situation."
He said the Government would give any support to the miners and their families.
"Our hearts and thoughts go out to them [affected families] at this time. It will be a very worrying time for them."
New Zealand Mining Minister Gerry Browlee said the Government would put whatever resources were needed into rescuing the miners.
"Their priority will be getting people out," he said. "I certainly feel very sympathetic to (their) families, it's a dreadful situation for them to be in," he said.
Blast rips through mine
An afternoon shift of miners and some management were underground in the mine when an explosion occurred around 3.45pm.
There was a power outage at the mine this afternoon, and an electrician went in to investigate about 3.50pm. He discovered a loader driver who had been blown off his machine about 1500m into the mine shaft.
He was one of the two men to later emerge from the mine. They had been working in a different area.
The two miners escaped by walking two kilometres in a valley inside the mine, climbing up inside the mountain. They eventually emerged through an escape portal.
The blast happened at 3.45pm and the last contact with any of the miners was lost at 4.15pm.
View Pike River Coal Mine, 586 Logburn Road in a larger map
Emergency services in place
Emergency services are at the mine's processing plant in Atarau, halfway between Greymouth and Reefton.
Six ambulances were on the scene, and three rescue helicopters were being sent from Nelson, Greymouth and Christchurch.
St John ambulance has more than 20 staff at the scene at the moment and is sending additional staff from Christchurch.
A crisis centre for victims of the explosion is being set up at the Greymouth Red Cross centre.
The Mayor of Grey District, Tony Kokshoorn, who rushed to the site of the blast, said "it's not good".
"With a bit of luck, it might be okay but there are 25 to 30 unaccounted for."
Rapid response mines rescue teams have been mobilised from their headquarters at Rapahoe, seven miles north of Greymouth.
Cause of blast unknown
New Zealand mining expert Dave Feikert said while it was unknown 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 in the mine in the production areas would be at the greatest risk.
If people were trapped underground, the mines rescue teams would probably ask for help from Australia.
"In the New Zealand mining industry, the miners are very well trained in all this but it doesn't mean they can't be caught out by a combination of circumstances," Mr Feikert said.
Escape tunnels in mine
Herald video journalist Simon Baker, who has been in the mine, says it does have an evacuation chute where miners can escape if the main entrance to the mine tunnel is blocked.
"They've built a small channel, which is just a ladder to the top of the mountain," said Mr Baker.
Mr Brownlee confirmed the existence of the exit tunnels but didn't know if they could be accessed by the miners.
He said he would be going to the mine tonight.
"Any assistance the Government can provide, the Government will be there."
Pike River Coal Mine
The mine is located 46 km to the northeast of Greymouth, halfway between Greymouth and Reefton.
The Brunner seam at Pike River holds the largest-known deposit of hard coking coal in New Zealand, with 58.5 million tonnes of coal in-ground. It runs six kilometres north-south and up to one-half kilometres east-west, averaging about seven metres in thickness.
The company had recently almost halved its production forecasts for the 2011 year to between 320,000 and 360,000 tonnes.
The mine is extremely isolated. It is a hill site mine that is made up of horizontal tunnels.
Around 150 people are employed by the mine. New CEO Peter Whittall started on October 2.