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11.31pm: Labour Leader Phil Goff has expressed grave concern at news of an explosion at the Pike River mine.
"Our thoughts and prayers are with the miners, their families who are waiting anxiously for news and for the West Coast community which is shocked by the incident and its potential consequences.
"We know that the Government and emergency agencies will do everything humanly possible to rescue those caught up in this disaster. We hope and pray the miners can be reached and recovered as soon as possible," said Phil Goff.
Labour's West Coast spokesman Damien O'Connor is at the mine site. He has also expressed his concern for those affected by the accident and sympathy for their families.
10.30pm: More than 20 St John personnel are at the scene and will remain overnight if required, a spokesperson said.
More paramedics have been sent to the West Coast by air and road in preparation for any rescue efforts which may take place tomorrow.
Superintendent Gary Knowles, the Tasman Police District Commander, is travelling from Nelson tonight 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," Mr Knowles said.
10.22pm: A Facebook group, Supporting the Pike River Miners, has been set up and is gaining more fans by the minute.
10.11pm: Specialist mine rescue teams and emergency services remain at the scene.
Air quality testing is being carried out because of the unknown atmospheric conditions underground, a police statement said.
Tasman police spokeswoman Barbara Dunn said the power was out in the mine.
Ventilation is the main concern, as electricity is needed to pump fresh air down that far.
9.59pm: Prime Minister John Key says the Government will give any support to the miners and their families it can.
"Our hearts and thoughts go out to them [affected families] at this time. It will be a very worrying time for them."
He understood there was a large explosion, but details of the size and scale were "scant".
"It has the potential to be a very serious situation."
9.50pm: West Coast DHB spokesman Bryan Jamieson says the injuries of two people brought from the Pike River mine are moderate, not life threatening and "consistent with the blast."
Both men,whose ages and names are not available, are both "walking and talking", police said.
9.35pm: Mine safety and energy adviser David Feickert says the mine rescue team will be "substantial", although he imagined only around six rescuers would venture inside.
Their progress could be slowed if there has been a methane leak, and they will be testing gas levels as they make their way down the mine.
"But it could be a race against time."
He said there was a possibility there could be another explosion.
"If ventilation is not working or not working well they will have to be quite cautious. They have breathing apparatus - but those won't save them from a gas explosion."
Mr Feickert said the mine should have two exits - just in case one is blocked.
"We don't know where the miners are so we don't know where they are in relation to the exits."
9.17pm: Rescuers are yet to enter the Pike River coal mine as they have to ascertain whether it is safe to do so.
Tasman Police communications manager Barbara Dunn told Radio New Zealand the major concern at the moment was air quality.
Mines Rescue, an organisation funded from parties throughout the mining industry, will be conducting the rescue operation.
The group specialises in rescue procedures in shafts and tunnels and is trained specifically to work in mine environments.
9.12pm: The rest of the world has picked up on the story of the Pike River mine explosion - CNN, Bloomberg, Al Jazeera, Reuters and more are all reporting the incident around the globe.
8.59pm: The two injured miners have now arrived at the Grey Base Hospital, TVNZ reports.
8.47pm: Pike River chief executive Peter Whittal says each miner has a safety kit including breathing apparatus with oxygen - enough to get to a safe area.
He also said the mine has not had any safety or maintenance issues recently.
8.45pm: Mine expert David Feickert says the safety of the rescuers is paramount.
"We don't know what exactly the situation is, but if the mine rescuers are about to go in it must mean it is safe for them to do so. They wouldn't go in if it isn't safe."
8.35pm: St John ambulance has more than 20 staff at the scene at the moment and is sending additional staff from Christchurch. Spokesperson Ian Henderson said it may be a long operation.
8.28pm: Mining expert David Feickert has told TVNZ that rescue workers will need to be on alert for another possible explosion.
8.15pm: A crisis centre has been set up at the Greymouth Red Cross following the explosion.
8.05pm: A rescue team is being briefed and preparing to enter the Pike River coal mine.
Newstalk ZB reported a Solid Energy crew had just gone in with stretchers.
8.02pm: Minister of Energy and Resources, Gerry Brownlee, will travel to the West Coast tonight.
Kate Wilkinson, the Minister of Labour and Conservation, will accompany him.
"The Government stands ready to offer whatever help and resources are required," Mr Brownlee said.
"It is important that the rescue authorities are given the space to do their jobs properly."
7.59pm: Peter Whittal says that unlike the recent high-profile Chilean mine rescue, emergency workers will be able to walk in and out of the Pike River mine through a shaft.
He said he knew all his staff personally, having individually recruited each one, and was extremely concerned for them.
"Not having communication with them is very distressing."
7.49pm: Canterbury District Health Board says it is making beds available at its hospitals in case they are required.
7.44pm: NewstalkZB is reporting it understands that a body has been taken to Greymouth District Hospital. However, a hospital spokesman said he could not confirm whether there were any fatalities and the CEO of Pike River, Peter Whittal said he had not received any reports of fatalities.
Whittal said he did not know if the remaining miners were trapped or choosing to stay below ground.
7.35pm: There are concerns that ventilation inside the mine shaft may be compromised by the power outage, police say.
7.25pm: Pike River chief executive Peter Whittal says 27 people remain unaccounted for - 15 Pike River employees and 12 local contractors.
He said two miners had left the mine "of their own volition" after making contact with the surface around 4pm and were treated for minor injuries.
One of the pair is the loader driver who was discovered by the
electrician who entered the mine to investigate a power outage.
The two miners indicated three of their colleagues were also on their way out.
7.15pm: Friends and family members have begun to gather at the mine, and have been allowed past the first police cordon.
EPMU secretary Andrew Little told CloseUp underground mining was inherently unsafe and the risk of gas explosions, particularly on the West Coast, was high.
While the industry was aware of the risks and took the necessary precautions, unfortunately these kinds of incidents still happened, he said.
Mining safety expert David Feickhert said methane was present in the coal seam at Pike River.