Among the concerns were pockets of "fire damp" or explosive methane gas in the mine, which could not be ventilated because the mine's fans were not working.
"We appreciate this is a very uncertain and worrying time for families and
friends of the miners and contractors who are at the mine. We are working closely with mine officials and other emergency services to do everything we can to help with the rescue operation," Superintendent Knowles said.
The rescue team was trying to make sure the vents were clear as there was a potential for a build-up of gas.
Until that happened, it was unsafe for rescuers to enter the mine, said police spokeswoman Barbara Dunn.
"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.
But Mayor of Grey District, Tony Kokshoorn was less hopeful, saying it could take days for rescuers to enter the mine.
Mr Kokshoorn also named one of the missing miners as Greymouth District councillor Milton Osborne.
He said Osborne was a contractor at the Pike River mine.
Waiting and hoping
Meanwhile families have been gathering at the site, waiting for news.
The 27 men consist of 15 local employees and 12 contractors.
Two men, Russell Smith, 50, and Daniel Rockhouse, 24, managed to escape the blast, and fled to the surface. 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 those miners never emerged.
CEO hopeful miners are safe
Earlier Pike River chief executive Peter Whittall remained optimistic the miners would be found alive.
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 said he 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 Brownlee 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 the 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.
Mr Smith and Mr Rockhouse walked two kilometres in a valley inside the mine, climbing up inside the mountain towards freedom. They eventually emerged through an escape portal.
The blast happened at 3.45pm and the last contact with any of the trapped miners was lost at 4.15pm, Mr Brownlee said.
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.
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 Fiekert 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 Fiekert said.
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.
- with agencies
