Emergency services rushed to Drayton open-cut coalmine, 13 kilometres south of Muswellbrook, about 8.15am after the explosion injured five workers, including the mine manager.

Accident investigators said the group had laid out five explosive charges as part of routine blasting but approached the area when one of the charges failed to blow.

When the group, which included the mine manager and two supervisors, were about two metres away from the blast site the unexploded charge blew in what Department of Primary Industries experts said was a highly unusual incident.

"It was a technical issue but it's such a one-off, rare, one in a million chance," the department's minerals spokeswoman Jenny Ward said.

"One of our investigators said that in his 25 years' worth of experience he has never heard of it before."

She said it was standard practice for miners to approach unexploded ordnance after waiting about 10 seconds.

Three of the injured workers were flown to John Hunter Hospital suffering internal chest injuries.

The men, all aged in their 40s, were in a stable condition last night, a spokeswoman for John Hunter Hospital said.

A fourth was taken to Muswellbrook Hospital as a precaution and was released, while the mine manager returned to work.

"The investigation will be now made up of us, the explosive's manufacturer and the mine," Ms Ward said.

She said mining explosives and safety protocols had been refined to the point where accidental discharges were highly unlikely.

But Construction, Forestry, Mining and Energy Union Hunter district president Peter Jordan said further investigation was warranted.

"They survived something that could have very easily killed them," Mr Jordan said.

"Drayton is very lucky that they got away with only these injuries and not five fatalities."