Around 30 workers were unaccounted for after an explosion ripped through a coal mine in New Zealand today, police and media reports said.

Emergency services were racing to the Pike River Coal Processing Plant near the town of Atarau on the west coast after police received reports of an explosion, a statement said.

Local Mayor Tony Kokshoorn told New Zealand's National Radio that around 30 people at the mine were unaccounted for.

"It's not good news at all," he said. "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."

St John Ambulance service said two rescue helicopters and 10 ambulances were heading for the mine.

The mine's management said up to 35 workers were underground when the explosion occurred, and that "two miners had come to the surface and spoken with them, saying that they had lost communication with the other miners", local media reported.

The mine reaches some 2.3km underground and produces high grade coal for export which is used primarily in steel production.

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

Pike River says its coal preparation plant at the site is the largest and most modern in New Zealand and processes up to 1.5 million tons of raw coal a year.

The mine is not far from the site of one of New Zealand's worst mining disasters - an underground explosion in the state-owned Strongman Mine on January 19, 1967, which killed 19 workers.