Rescuers are concerned about air quality at the site of today's Pike River coal mine explosion on the west coast of the South Island, which is believed to have trapped 27 people.

Police said there was a concern that ventilation inside the mine shaft may be affected. The mine's main ventilation shaft, 110m deep, partially collapsed in February 2009.

Air quality testing is being carried out because of the unknown atmospheric conditions underground.

Police said the two miners who walked to the surface earlier this evening were being treated for moderate injuries at Grey Hospital. Both men, whose ages and names are not available, are both "walking and talking".

Specialist mine rescue teams and emergency services remain at the scene, 50km north east of Greymouth.

Superintendent Gary Knowles, the Tasman Police District Commander, is travelling from his Nelson Headquarters to oversee the police response.

Police, including Search and Rescue specialists, are also gathering in Greymouth and at the scene which is cordoned off to the public.

"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.

A welfare centre has been set up for the families of miners trapped by the underground explosion.

Police said the centre, at the Red Cross Hall in Greymouth, is a warm and dry gathering point for family members as they anxiously await news from the mine site.

Red Cross and Victim Support volunteers are at the centre providing comfort and refreshments.

Families are encouraged to use the welfare centre instead of trying to reach the mine's access road which is closed to all but those involved in the rescue and recovery operation.

At least 27 miners are still unaccounted for - 15 mine employees and 12 contractors.

.The miners appeared to have been trapped following an explosion at the isolated underground mine, 50km northeast of Greymouth, this afternoon.

An electrician went into the mine to investigate a power outage about 3.50pm. He discovered a loader driver who had been blown off his machine, about 1500m inside the mine shaft, police spokeswoman Barbara Dunn said.

The mine entrance is about 2.2km long, and then branches out into other areas.

Thirty-six tags belonging to miners were still hanging on the board at the entrance to the mine.

One of the pair to emerge was the loader driver, and they indicated three more workers were on their way out although there was no sign of them by 8pm.

The two men were flown by rescue helicopter to Grey Base Hospital with moderate injuries.

The mine rescue team was trying to make sure the vents were clear as there was a potential for a buildup of gas. Until that happened, it was unsafe for rescuers to enter the mine, she said.

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.

The mine started shift early in the afternoon and lost communication about 3.45pm, Pike River chief executive Peter Whittal told Campbell Live.

The mine was alerted at 4.10pm that something was wrong.

The missing miners could be trapped or choosing to shelter underground, he said. He could not confirm a report that one person was dead.

"I personally know every employee of the company ... I know what the shift is and who men generally the men are on that shift, and I'm still waiting to get a full list of the employees involved,'' he said.

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 said.

"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.

New Zealand's largest coal miner, state-owned Solid Energy, said Pike River had requested first aid equipment.

The gear would come from the company's underground Spring Creek mine in the hills behind Dunollie, north of Greymouth. Solid Energy also had workers involved with Mines Rescue.

A Department of Labour mines inspector based on the West Coast was at the mine to provide assistance and advice, and Energy Minister Gerry Brownlee was also heading to the mine.

Ikamatua panelbeater John Batten said everyone in the town - about 15km from the mine - was very concerned.

"Everybody's hoping for the best and it's not looking good at present. It's an absolute major. We're just waiting,'' he said.

"It's catastrophic.''

Some people from the town worked at the mine but not underground. The Ikamatua volunteer fire brigade was among the emergency services at the mine.

"We all know everybody,'' Mr Batten said.

(Source Otago Daily Times)