An explosion has ripped through New Zealand mine while 30 miners were inside.

Two miners managed to escape the Pike River Coal Processing Plant, near the town of Atarau on the west coast after the blast at about 3:45pm local time.

A further three are believe to be on their way to the surface, leaving 27 miners still unaccounted for.

Emergency services scrambled to the site where the trapped miners are estimated to be up to 1500m underground.

An electrician went into the mine to investigate a power outage about 3.50pm and discovered a loader driver who had been blown off his machine about 1500m inside the mine shaft, police spokeswoman Barbara Dunn said.

One of the pair to emerge was the loader driver, and they indicated three more workers were on their way out although there was no sign of them by 8pm and no communicatiion with them could be made.

Emergency services, including three rescue helicopters and six ambulances had race to the scene and emergency exit tunnels have been built into the mine but it is unclear whether any of them can be accessed by the miners.

Police said there was a concern that ventilation inside the mine shaft may be affected. The mine’s main ventilation shaft, 110m deep, had partially collapsed in February 2009.

The mine rescue team was trying to make sure the vents were clear as there was a potential for a buildup of gas. Until that happened, it was unsafe for rescuers to enter the mine, she said.

Local Mayor Tony Kokshoorn told New Zealand’s National Radio “It’s not good news at all”.

“We don’t know at what depth the explosion is but there’s certainly a big explosion. With a bit of luck, things might be OK. But there’s 25 to 30 miners unaccounted for.”

Pike River Coal chief executive Peter Wittall said the two miners were being questioned to ascertain the full nature of the incident.

“We’ve had our afternoon shift underground and we’ve had communications with a couple of the employees and we’ve had two men return to the surface who are being interviewed.”

“One of the employees has said he felt an explosion underground and since then he’s walked from the mine with another employee. We have no further information at this stage,” he said

The coal seam is about 200 meters underground and is reached through a horizontal tunnel 2.3km in length. One vertical ventilation shaft rises 108m from the tunnel to the surface, according to the company’s website. This was blocked by falling rocks within the shaft in early 2009, delaying mining for months.

Pike River has been operating since 2008, mining a seam with 58.5 million tons of coal, the largest-known deposit of hard coking coal in New Zealand.

The cause of the blast has not yet been identified, but Pike River Coal chief executive Peter Whittall — who oversaw much of the development of the company’s mine 46km northeast of Greymouth — started the week promoting the mine’s prospects to shareholders who went to the company’s annual meeting at the mine site.

Methane gas has long been identified as a potential problem for the Pike River Coal mine.