Rescuers cannot enter the West Coast mine where at least 27 miners are trapped because of poisonous gases.
Dozens of emergency service workers, including a specialist mine rescue team, rushed to the Pike River Coal Mine, at Atarau between Greymouth and Reefton, about 4.30pm on Friday after an explosion.
Two miners are being treated for moderate injuries after the blast, the cause of which is not yet known.
The other 27 miners have been trapped below ground with no communication to the outside world since the explosion.
Pike River chairman John Dow says the rescue team cannot go inside to get them until the team is certain it is safe.
Tasman Police District Commander Gary Knowles says rescuers are concerned about air quality and gases in the mine. Air quality testing is being carried out at the site.
Superintendent Knowles travelled from his Nelson headquarters overnight to oversee the police response.
He says more police, including Search and Rescue specialists, are gathering in Greymouth and at the scene.
Superintendent Knowles says he appreciates this is a very uncertain and worrying time for families and friends of the miners and contractors who are inside the mine.
But he says it is not not expected police will have any further updates until Saturday morning.
Ventilation concerns
Tasman police communications manager Barbara Dunn says there are great concerns with no power to run the ventilation system in the mine shaft.
Ms Dunn detailed some of the events after the blast.
An electrician went into the mine at 3.45pm to investigate a power outage and found a loader driver who had been blown off his machine about 1500 metres into the mine shaft.
The loader driver and another miner made their own way out. A St John spokesperson says they are in a stable condition in Grey Base Hospital with moderate injuries.
Ms Dunn says they indicated three others were on their way to the surface.
The mine company says all the trapped miners and contractors are equipped with self-rescue gear.
Scene near mine
A Radio New Zealand reporter says the road into the mine is blocked off by a police cordon about 12 kilometres from the township of Ikamatua.
The only vehicles being allowed in are those driven by emergency services.
About 11.30pm, three police cars and a search and rescue van drove past the cordon to join dozens of other emergency staff waiting at the mine entrance.
Energy and Resources Minister Gerry Brownlee has also arrived at the mine, joining Grey District mayor Tony Kokshoorn.
Some worried family members have been standing vigil at the police cordon.
Mr Kokshoorn earlier told Checkpoint police had told him the situation was very serious.
Listen to Tony Kokshoorn on Checkpoint
Communications lost
Pike River chief executive Peter Whittall says the missing miners are about 120 metres beneath the surface, and believed to be in the mine's roadways.
He told Radio New Zealand News earlier in the night one miner who reached the surface confirmed an explosion had occurred.
"He believed there was an explosion, and since then he and another employee have walked out under their own steam. They're still being interviewed as to where they were, and what they heard and saw at the time.
"The rest of the employees, because we lost communications underground when the incident occurred, we've not been able to communicate with them to understand exactly who is where in the mine."
Joint response
By evening, St John Ambulance had six ambulances and 20 staff at the mine and was preparing to bring in additional staff from Christchurch to assist with what it says may be a long operation.
Initially it called in three rescue helicopters from Nelson, Greymouth and Christchurch.
The Fire Service says a command unit from Christchurch is on standby, with three appliances at the mine.
Greymouth hospital's emergency plans have swung into action.
Grey Base Hospital says nursing staff shifted patients to ensure there are adequate bed numbers in particular areas, and extra staff arrived to prepare for the expected work to come.
Spokesperson Bryan Jamieson says the hospital regularly has planning meetings for multi-casualty emergencies which might happen, such as a mine explosion.
A welfare centre has been set up for the families of the trapped miners.
The centre, at the Red Cross Hall in Tainui Street, Greymouth, will provide a warm, dry haven for family and friends as they await news from the mine.
Government pledge
The Government says it will offer any assistance needed to help rescue the trapped miners.
Prime Minister John Key says the situation has the potential to be very serious.
He says the Government's thoughts also go out to the relatives of the trapped miners.
Conservation Minister Kate Wilkinson is going to the scene, as the mine is on Department of Conservation land.
Gas risk
The coal miners' union says gas explosions are a constant risk in West Coast mines.
Engineering, Printing and Manufacturing Union national secretary Andrew Little says early reports indicated a major incident. The union has 70 members at the mine.
Mr Little says Pike River has good health and safety procedures.
Brunner coal seam
The Pike River Coal Processing Plant is in the Paparoa Ranges 50km northeast of Greymouth. The mine entrance is about 2.3km long and then branches into sub areas.
On its website, Pike River says it broke through to coal in October 2008 when the tunnel intersected the Brunner seam of premium hard coking coal.
It holds the largest-known deposit of hard coking coal in New Zealand, with 58.5 million tonnes of coal in-ground.
The mine has secured in Japan and India, with India buying 20,000 tonnes in the first export shipment in February 2010.
Pike River coal, the website says, is sought after because of its special qualities including the world's lowest ash content for a coking coal, meaning more energy and less waste in the coke making process.
