The GAG jet engine started up at the troubled mine just over a month ago, and has never faced such a long tour of duty.

"We are setting new precedents in regards to the running of the GAG unit for this length of time," Hartley told ILN.

He said the engine was running well, but there were natural concerns about the life of the jet engine as it has been going for more than 600 hours.

"Jet engines run far in excess of that but there are safety margins," he said.

The turbine of the jet engine has been put to the test, but so have the GAG crews who have been working around the clock right through Christmas and New Year's Day.

"We want to bring it to a close so we can get our crews back because they have been there for a long time," Hartley said.

The experience has provided QMRS with insight into running the GAG for prolonged periods.

Managing the "fatigue factor" for crews and trying to align maintenance schedules with "strategic decisions" are some of the lessons learned, Hartley said.

But the operation of the GAG could continue for many weeks yet, despite the expense.

The jet engine consumes around 1800 litres per hour at an average run time of 20 hours per day.

Hartley said the fuel flown over from the New Zealand Air Force cost about NZ60c per litre.

Terrain and weather

The young Pike River mine was a small but complex operation. Its location in mountainous and isolated terrain is creating a host of difficulties for the recovery effort.

Access around the mine is limited and, because everything has to be done by helicopter, efforts are dependent on weather conditions.

Hartley said the supplementary gas monitoring systems put in place after the initial explosions were also vulnerable to the weather - not just heavy rain but the wind and falling trees along with rock and earth movements.

Gas monitoring

Hartley's last visit to New Zealand was after Christmas for a 48-hour trip and his direct involvement in the recovery operation provides insights into the mine before tragedy struck.

The level of gas monitoring before the first explosion will certainly be subject to the inquiries underway, but Hartley confirmed that measures were in place before the disaster.

"Very few mines have real-time [gas monitoring] - it's all tube bundling systems, they had fixed systems, sampling systems yes," he told ILN.

He added that the mine used Simtar's Cgas Pro software system, which does the same job as its better-known product Camgas.

Hartley said the gas-monitoring systems were wiped out from the first explosion.

"They had gas monitoring, I don't know whether they had a GC [gas chromatographer] onsite," he said.

"I believe they did, but I can't confirm that.

"I believe they did have a GC onsite, I believe they had gas monitoring but all the gas monitoring sampling lines and tube bundles, those sort of lines, got destroyed in the event."

With no option to enter the mine since the explosions, a tube bundle system was brought over from Queensland and an overland gas monitoring system was established using boreholes and shafts.

"That is the best sampling you are going to get," Hartley said.

While some readings have registered zero methane as the GAG operated, the exposed coal underground is causing more methane to accumulate to the back of the mine.

Hartley said grading for this area was up to 1 in 8, meaning an 8 metre rise for one metre of distance, making it "extremely difficult to get methane out of there"

"That is a very steep grading and the methane will migrate to the very highest level," he said.

Pike River unknowns

Noting the speculation on the temperature variations in the mine, Hartley outlined some factors at play.

"If you have a number of explosions in the mines, you will have a heating of coal, there was a fire in the mine so we can put inert gases into the mine," he said.

"We can control the flames, stop the flames propagating, [but] we will still go through slow continuous heating until we can hold it that way for some time.

"We won't know exactly what we got until we get back into the mine."

He also commented on the interest in the cause of the first explosion in the mining tragedy which claimed 29 lives.

"Everyone is speculating on who, what, where and why.

"But I think just allow the investigative process to go forward and we will all learn from this and New Zealand will be a different place after this."