The devastated families of the Pike River 29 are being urged to cling to hope.

It is a slim hope: to find survivors from the massive methane explosion that tore through a West Coast coal mine.

As relatives waited on a knife-edge to learn of news of their loved ones early today, a source close to the rescue effort said emergency services were not confident.

Search and rescue teams had agonised over sending a team into the mine, but were held back by potentially deadly levels of poisonous gases swirling in the pitch-dark Pike River tunnels.

Authorities have discussed sending a robot down into the 108m shaft, after repeated calls to a land line and mobile phone in the mine went unanswered.

A man close to the rescue team told the Herald on Sunday early this morning that the source of the explosion was in an outer part of the mine, and it blew inwards.

Only two men have escaped the mine, by climbing up the vertical shaft. Those two reported that three others were following close behind - but they never emerged. Noxious gas levels in that shaft were said to be high.

Police would not comment early today. But two international mining experts spoke out about safety concerns at the mine, and told how it had been flooded by methane gas just three weeks ago.

Prime Minister John Key, who visited the stricken families at a Red Cross welfare centre in Greymouth yesterday, said they were showing signs of "genuine fear".

He said: "This is a time of huge anxiety and concern for the families and the miners, so our hearts and thoughts go out to them."

Key encouraged the families to take hope from inspiring tales of survival such as that of the trapped Chilean miners.

"They have to hang on to hope. The rescuers are brave people but they could make the situation worse by going into the mine and igniting another explosion."

Prince William emailed the prime minister to express his concern, and Australia's prime minister Julia Gillard sent a message of support.

More than 200 people filled the Holy Trinity Anglican Church in Greymouth last night for a special service to help people cope as they wait for news.

Vicar Marge Tefft said the large turnout was an incredible display of "uncertainty, concern, and sorrow and hope".

She told the congregation she had noticed how people were helping each other and giving support and "doing practical things" for others in the time of need.

Archdeacon Robin Kingston said he wished he could say it was unfamiliar territory, in reference to the mining tragedies that have hit the West Coast in past years.

"Coasters have experienced so many tragedies over the years, you would think we are hardened by it. But we never are."

Special services were planned at churches in Auckland, Wellington and Christchurch today.

Australia was sending a team of rescue experts to assist, Foreign Affairs Minister Kevin Rudd said.

Earlier in the evening, the police officer in command of the rescue effort said he had no option but to hold back the rescue team, consisting of experienced miners, many of whom had family inside in the mine.

District Commander Gary Knowles said: "I'm not going to put anyone else underground until I'm satisfied that it's safe and we're going to get people out."

Although they repeatedly referred to the operation as "search and recovery," Knowles and the mine's chief executive, Peter Whittall, insist they can find men alive.

"I'm the eternal optimist; that at the end of the day we're going to find these guys and bring them out," Knowles said.

The rescue attempt could begin as soon as tests showed the gases - including methane, ethane and carbon monoxide - were clearing.

"We don't work on gut feelings," he added.

Whittall, showing the strain of more than 24 hours without sleep, said the men's families were drawing comfort from one another at the local Red Cross emergency centre.

"They're spending a lot of time with each other and there's not much they can do," Whittall said. "It's much like the rest of us, they can only wait."

A source at Valley Longwall Drilling, a company contracted by the mine, said the more time they spent underground, the less likely they were going to be pulled out alive.

"The problem is the air is no good - when there's an explosion the flame sucks up the oxygen so we just don't know what they'd be breathing down there," the source said.

AMONG THE MISSING IN PIKE RIVER MINE

* Milton Osborne
* Ben Rockhouse
* Conrad Adams
* Zen Drew
* Brendon Palmer
* Pete Rodger (Scotland)
* Josh Ufer (Australia)
* Koos Jonker (South Africa)
* Michael Monk
* Kane Nieper
* Blair Sims
* Malcolm Campbell (Scotland)
* Chris Duggan
* Alan Dixon

Reporters: Anna Leask, Kieran Nash, Rebecca Lewis, Leigh van der Stoep, Abby Gillies, Michael Dickison, Jarrod Booker, Joanne Carroll, Claire Harvey, Catherine Masters, Bevan Hurley.