A scientist says temperatures inside the Pike River mine may have soared higher than 1200C as a fireball ripped through it.

"Initially some parts of the mine could be very hot - the peak temperature of the explosion would be of the order of 1500 degrees Kelvin (1227C)," said Associate Professor David Cliff at Queensland University's minerals industry safety and health centre.

Asked if there was a threshold in mining disasters like this after which the probability of miners surviving an underground explosion really started to decrease, Professor Cliff said: "The clock ticks from the time of the explosion."

A mining expert says methane in the Pike River mine could have built up to explosive levels in a matter of hours but is surprised alarms did not warn workers before the blast.

Workers have previously left the mine when methane levels reached a potentially dangerous level.

University of Canterbury geology senior lecturer David Bell said his department had studied rocks from the mine to assess whether sparks would be produced as boring machines smash through them.

Although it was not known whether a spark from the road header drill was the cause of the explosion, it was one possibility, he said.

Pike River's chief executive, Peter Whittall, said the mine was not unsafe and the company had systems to deal with hazards.

"We are not an unsafe mine. However, an unsafe condition occurred, otherwise something wouldn't have happened. What caused it, we don't know. What the situation was, we don't know."

The mine had just completed its annual risk review for insurance purposes. It had highly qualified, highly experienced staff, Mr Whittall said.

Mr Bell said the upper-lower explosive limit was 4 per cent methane in the atmosphere and the upper limit 14 per cent. It was thought levels were about 6 per cent to 8 per cent around the site of the blast, he said.

"Ventilation is the primary solution. Clearly there's been a gas buildup - maybe they've been driven into a pocket in a fault they hadn't identified."

Although gas can build quickly, he said he was surprised monitoring equipment had not picked it up.

"Something has got out of hand that hasn't been detected in time."

The Labour Department yesterday said it had started an investigation into Friday afternoon's explosion but could not comment on whether there were any safety concerns at the site.

"Our investigation of the incident will review all interactions with the company and until it is complete we cannot release any detailed information about them," a department spokesman said.

Engineering, Printing and Manufacturing Union secretary Andrew Little said at the weekend there had "been pretty standard procedures in place and nothing ... that would have pointed to a potential risk was raised by workers".

State-owned Solid Energy's Spring Creek underground mine, about 30km from Pike River, was shut yesterday because a number of staff were involved in the rescue operation.

* Pike River says the mine was not unsafe but "an unsafe condition" happened before the blast.
* The Labour Department yesterday launched an investigation.
* It has had "regular engagements" with Pike but refuses to comment on the safety record.
* A union boss says workers were not concerned about extra risks.

- additional reporting by NZPA