Pike River Coal CEO Peter Whittall says it unlikely that yesterday's explosion could have caused a collapse at the mine, where 29 miners remain missing.

He expects the mine's structural integrity to hold due to the design and materials used.

"Mine explosions don't typically damage the roof unless there are timber props that can be knocked out," he said at a press conference this morning, "we don't have those in the mine."

"We've got a very strong roof, it's all fully bolted and I would believe it's unlikely that an explosion even of a significant magnitude would actually damage the [mine shaft]," said Whittall.

He said that as the mine is a tunnel mine, not a shaft mine, which means a rescue will be much more easy, once it is deemed safe to enter.

Officials remain concerned about the possible levels of methane gas in the mine.

Mr Whittal said they weren't aware of any hazardous circumstances in the mine on Friday and added the miners have an oxygen device which can give them 30 to 60 minutes of clean air.

Whittall said he has assured family and friends of the trapped miners that the company is doing every humanly and technically possible to recover them.

"It's very much a kingship and brotherhood in the mining industry," he said.

"I have assured them we are doing everything that's humanly possible and technically possible to recover their families and friends."

"The families are very obviously distressed. No knowledge is a bad thing in these situations. There was very intermittent knowledge coming through overnight."

Families gathered outside the Pike River Coal offices in Greymouth early this morning to hear the latest news of the 29 trapped miners.

Looking tired and anxious, the families are hearing a briefing from Pike River Coal and rescue authorities who are still not able to enter the mine after the explosion yesterday afternoon.

Grey District Mayor Tony Kokshoorn has been greeting each family member with hugs.

Kokshoorn said the successful Chilean mine rescue is giving him hope.

"We remember the Chile mine recently, that every miner got out. At the moment, we're hanging onto every hope, just like they did.

However, Mr Kokshoorn said a speedy rescue was not likely.

"Rescue crews are doing all they can, but this is going to be a very slow recovery."

Rescuers are still waiting for the all-clear on gas build-up before going into the coal mine.

Nothing had been heard from the men since the explosion deep underground yesterday afternoon.

Worried families rushed yesterday to the entrance to the Pike River Coal company's mine, 46km east of Greymouth.

The alarm was raised when mine electrician Russell Smith, 50, investigated a power cut at 3.50pm.

He discovered loader driver Daniel Rockhouse, 24, who had been blown off his machine.

The two men, who were working some distance from the other mine staff, escaped through a service portal and walked 2km through a valley to alert emergency services.

The pair had moderate injuries and were taken to Grey Hospital in Greymouth.

Mr Smith was last night back at home, but his partner, Donna Ryan, said he was in no state to talk.

"We're so relieved. He's just come home and is resting up in bed."

Christchurch-born Mr Rockhouse, 24, is married with a 3-year-old daughter and has worked for Pike River Coal since June 2008.

A rapid-response mine team, police and staff from six ambulances and three rescue helicopters were at the mine early today, but police said the rescuers were worried about air quality inside and were waiting at the entrance.

Superintendent Gary Knowles said air-quality testing was being done because of the unknown atmospheric conditions underground.

One of the missing miners is Grey District councillor Milton Osborne.

His son said last night the family were waiting for news. "They're not out yet. We haven't heard anything."

Pike River Coal chairman John Dow earlier refused to comment on the fate of those trapped.

"We're hoping for the best, but we don't have enough information to speculate at this point," he said.

Last night, after saying that power had been restored to the mine, he said he did not know when the rescue team would enter the mine, as they were still assessing the safety risk.He said it was a traumatising time.

"It is absolutely awful. It is hard to describe, because these types of events happen so rarely and we don't know what happened."

"With the power not working, and therefore the ventilation not working, they're still checking if it's safe for people to go in the mine.

"Obviously in a coal mine like this, when the ventilation goes off, gas builds up in the mine."

Mr Dow said each miner carries safety equipment, including a self-rescuer - a device which gives miners oxygen for up to an hour.

Mr Dow said these self-rescuers are also gathered in several caches throughout the mine.

"They don't need the power to be restored before they can go in."

Mr Dow said the miners might have made their way to one of several safety refuges, where extra air supplies were available.

"There can be air in the refuge even if there is gas in other parts of the mine."

But the supply was not fresh air and it could run out.

Pike River Coal chief executive Peter Whittall said the missing men were probably between 2km and 2.5km into the mine, although because the mine drills into the side of a mountain, they were probably only about 120m below the surface.

He said a coal-gas explosion was the most likely cause of the blast.

He said he could not confirm a report that one person was dead.

Mr Whittall said the mine shift started early in the afternoon and communication was lost about 3.45pm.

Mine management was alerted at 4.10pm that something was wrong.

Fifteen of the missing men were mine employees and 12 were local contractors.

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

Veteran West Coast journalist Paul McBride, 56, of the Greymouth Evening Star, said that after looking at the damage he captured on film, "I would say the outcome will not be good".

The mine entrance looked "reasonably normal", but the escape portal showed obvious signs of a "pretty big fire and explosion".

McBride surveyed the area from a helicopter just before a no-fly zone was established.

Prime Minister John Key said last night the Government would give any support required to the miners and their families.

Greymouth High School principal Jim Luders was waiting for a clearer picture before calling together senior managers to discuss who among his pupils would need counselling.

He said 25 families with children at the school would have connections with the mine.

"We are quite nervous, because a lot of our families are connected with the mine and we have former students who have headed off from the school to work in the mine.

"They are real characters, great young students. They love working in the mine environment and wanted to do it for a couple of years.

"Everyone is quite connected because the mine is important here and it's a tight community despite the fact they are from all over the world.

"They are South African, Australian and English as well as Greymouth families."