An attempt to rescue 29 Pike River miners, trapped underground since an explosion in the coal mine yesterday afternoon, is dependent on air quality tests - and could happen as late as tomorrow.

"Until safety regulations are met, we are not going to speculate.

"It could happen overnight, it could happen in the next hour," said Tasman Police District Commander Gary Knowles at a press conference.

Rescue teams were prepared to go into the mine at any hour of the day, he said.

He said the focus today had been on search and recovery, however rescuers have not had the opportunity to go underground.

"We are still waiting for a window of opportunity where we can do this. As the search commander I am not prepared to put people underground until we can prove it is a safe environment," he said.

"We still remain positive, and we believe that once that window of opportunity opens we are ready to go."

Mr Knowles said the rescuers have been focusing on getting their equipment ready, going through drills and considering the various risks posed.

"When this window opens, we only have a short time frame to get in there, look at what's down there and make a decision on what we are going to do."

Mr Knowles said the rescue helicopter had been used four times to transport air samples from the top of the shaft to Rapahoe.

Knowles earlier said that four air quality tests taken over the mine's vent shafts had been completed.

The tests were to determine whether the air composition in the Pike River mine was safe enough for a rescue team to move, but have been inconclusive, police said.

Police are still refusing to give the rescue team the go ahead.

Officials fear a rescue effort could trigger a second explosion and will not allow a team to move until test results prove combustion is unlikely.

Police will hold another press conference at 8am tomorrow.

PM says global support 'huge'

Prime Minister John Key said everything would be done to ensure the "29 brave men" were taken out of the mine.

"This is a time of huge anxiety and concern for the families...our hearts and thoughts go out to them."

He said expressions of concern were pouring in from around the world in "huge" numbers.

"Prince William personally sent me an email before to say his heart and thoughts go out to the miners".

The big issue now was balancing the rescue of those trapped with the safety of rescue workers, he said.

"It's a difficult time for everyone, but we are determined to get the men out alive".

Pike River CEO Peter Whittall said special testing equipment is being flown in after contamination problems.

"At the moment we've had to fly [gas samples] from up in the mountain down to Rapahoe, which is a small town just up the coast, to have the gas analysed," he said.

"Earlier one of the issues was gas leakage and contamination so what we're doing about that is getting a gas chromatograph.

"Once we've done that we can do our gas bag samples on site more easily, much quicker and more reliably," Mr Whittall said.

Whittall also noted that there were only "several hundred metres of mine road ways".

"So when the rescue teams go in ... they will be looking to come from the tunnel right up to this pit bottom area near where our shaft is and establish a fresh air base and from there they can do a search and recovery of the rest of the mine.

Whittall said it is not a large area and the men will be close to each other even if they were working in separate areas.

Safety of rescuers vital

Pike River chairman John Dow also said there would be no rescue attempt until mine officials were certain it was safe to go in.

"You don't send rescue teams underground until you are sure of the environment in which you are sending them.

"They need to know what the mix of gases is in the mine air, so they don't run the risk of triggering another explosion which might endanger the lives of rescuers and jeopardise the lives of miners that might be safe underground," he said.

"We got a sense of what the air quality was like last night, but there's concern surface air may have been mixed with that coming up the shaft because we were only able to take the sample from the top of the vent shaft."

Kokshoorn 'misunderstood by media'

Grey District Mayor Tony Kokshoorn said around midday that a rescue attempt would be made "shortly", but Dow said he had been 'misunderstood by media'.

Energy Minister Gerry Brownlee said he was unaware of the all clear being given for a rescue attempt.

"Basically the position I've taken is with the people who are at the site that when they are doing something, they let me know," Mr Brownlee said.

"I don't know where he (Kokshoorn) got his information from."

"The weather is unbelievable here. I'm looking out the door of the building I'm in and it's very low cloud, so I would be surprised if that is correct."

The mine rescue team had already undergone a briefing as they prepare to go down to the mine.

Waiting game

As rescuers waited for the go-ahead to enter the stricken mine, Kokshoorn was clinging onto hope that the 29 miners trapped underground will be found alive, but admitted that "every hour that goes by, it gets more dire".

"We are being realistic. There's been a major accident," he said.

"It's chaotic at the moment, but what's frustrating is the rescuers can't go in."

Anguished families have gathered, desperate to hear any information about their missing loved ones.

While many of the miners are thought to be locals, several of the trapped are believed to be from South Africa, Australia and the United Kingdom.

Devastated community

Kokshoorn said the community was "grief stricken".

"One of my councillors is down there. A young fella who got the West Coast rugby league player of the year last week is down there, a local publican - his son is down there."

A father of five young children was also named as one of the men unaccounted for. The youngest person is a 17-year-old assistant and the oldest, a 62-year-old man.

Pike River Coal CEO Peter Whittall told a press conference this morning that concerns remain over air quality and potentially explosive gases in the mine.

"In six to eight hours we will have enough analysis as to whether the rescue team can go underground."

He also assured family and friends of the trapped miners that the company is doing every humanly and technically possible to recover them.

"Our primary focus over the next 24 hours is rescue and recovery. Before we put rescue teams down want to make sure environment is stable," Whittall said.

Whittall said damage to the upper part of mine shaft in the explosion meant "we can't get accurate samples."

"The only way we can determine what is happening underground is to monitor the air coming to the ground."

"We can't tell the absolute quality of that, so we can't risk sending anyone into the mine."

"We are very confident once we can get some bag samples we should be able to get some rescuers underground."

Hazards that the trapped miners have to deal with will include air pollution, high methane levels, high carbon dioxide and low levels of oxygen.

"There is a fresh air base underground. One of miners went into it before he came out.

"There could well be men in that chamber underground but at this stage we don't know," Mr Whittall said.

Specialist gas testing equipment was flown in from Australia overnight.

We are going to bring these guys home

Tasman District commander Superintendent Gary Knowles expressed strong determination to get the men out of the Pike River mine at this morning's press conference.

"This is a search and rescue operation," he told media, "and we are going to bring these guys home."

Police have confirmed 29 men aged between 17 and 62 years are trapped in the mine, including 16 Pike River employees and 13 contractors.

Family support

The message that Whittall is sending families and friends of the trapped miners is that this disaster is a "shared experience".

"I know all of the men, I've employed all of the men. It's very much a kinship and brotherhood in the mining industry," he said.

"When I look down the list [of those trapped] I can see the faces of everyone on the list," he said.

"There's a brother of one that got out, there's a son underground of one of the mining officials. These are men that everyone in the room knows."

"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."

All next of kin have been notified, he said.

Mine integrity

Whittall noted that it was unlikely that the 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 tunnel," said Whittall.

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

Mr Whittall 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.

Chile rescue offers hope

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.

Tony Kokshoorn was earlier greeting each family member with hugs. He 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.

Two miners out

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

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.

"One of them talked about being knocked to the ground. He believed he was knocked unconscious by the blast," Mr Whittall said.

Both of the men were working alone at the time and one of them was driving a vehicle about 700 metres into the mine.

The other one was a couple of kilometres in, Mr Whittall said.

Mr Whittall said both men were "shaken up, bruised and disorientated" and it "took a lot for one of the men to get to the point to explain what happened", Mr Whittall said.

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.

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."

Local knowledge

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.

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