The GAG unit flown for the Pike River coal mine recovery effort is "no gimmick" and has been used successfully in Queensland mines, the head of an Australian miners' union says.

Officials are trying to make the Pike River mine stable enough for rescuers to recover the bodies of 29 men trapped after an explosion a week ago.

Despite a brief stabilisation in the mine on Tuesday night, the 2.8km network of tunnels has refilled with potentially explosive methane and poisonous carbon monoxide.

The second enormous explosion on Wednesday afternoon briefly burned away some of the hot gases but with no working ventilation in the mine shaft further blasts cannot be ruled out.

The Royal New Zealand Air Force flew the "GAG unit" and 16 crew from the Queensland Mines Rescue Service into Hokitika at 6.30am today and is now at the mine.

The service's NSW state manager, Seamus Devlin, told Radio New Zealand the GAG unit would take three days to set up but could be ready for use on Monday.

Forestry, Mining and Energy union spokesman Steve Smythe told the broadcaster the engine had been used successfully in Queensland mines.

The "GAG unit" will stabilise Pike River mine by sucking all the oxygen out of the atmosphere, he said.

"This is not a gimmick. People are going to be a bit sceptical on the back of the robots and everything. But I believe they will be able to stabilise that mine."

When fully assembled, the unit would be 12m long and 2.5 tonnes, Mr Smythe said.

He expected there to be some "teething problems" before the unit started stabilising the mine and said there was no way of telling how long the process would take.

Recovery teams would need to do a full risk assessment before deciding where to place the machine, Mr Smythe said.

"It's not as straightforward as putting it at the entrance to the mine."

Queensland Mines Rescue Service manager Wayne Hartley earlier explained how the unit worked to Radio New Zealand.

"It's used for controlling underground coalmine fires particularly by inerting the atmosphere and displacing explosive gases or methane gases, extinguishing any fires and suppressing any sparking or sources of ignition," he said.

The Gorniczy Agregat Gasniczy engine pumps gas with very low concentrations of oxygen that will not sustain a fire in the mine. Steam is also pumped in. Over time this starves the fire of the oxygen and smothers it.

In 2003 a similar engine was used at the Loveridge Mine in West Virginia to put out a fire that had been burning 200m underground for two months. It took 10 days of continuous use but was months quicker than letting the fire burn itself out.

Coal fire risk

Pike River CEO Peter Whittall yesterday said that the Pike River mine was still an unstable environment and action needed to be taken swiftly to ensure a coal mine did not break out.

Tests taken through a borehole into the mine's main tunnel indicated that gas fires were burning inside. If the flames spread to the coal seam, emergency services could face a month-long battle with fire before a recovery mission could take place, he said.

Engineering and mining geologist David Bell said the worst coal fires could burn for decades. The Strongman 2 mine, also on the West Coast, burned for eight years before Solid Energy could extinguish it.

Starving the tunnel of oxygen would not quench a coal fire, because it could exist as heat for a long period before re-igniting again.

Even if a coal fire does not occur, officials have said the wait for recovery is likely to be several weeks.

Starving the tunnel of oxygen would not quench a coal fire because it could exist as heat for a long period before re-igniting.

An "engine" could force gases such as nitrogen or carbon dioxide into the mine to stabilise the atmosphere. Over time this starves the fire of oxygen and eventually smothers it.

Difficult at Pike River because the tunnel slopes uphill from the entrance. There is a possibility that water could be poured through the newly drilled borehole. However, this tactic makes the recovery of bodies difficult.