Two high-pressure water hoses have been replaced on the GAG inertisation unit, which was restarted at lunchtime yesterday and continued overnight and this morning.

A New Zealand Police spokesperson told ILN that gas samples were still “very variable” and confirmed that methane was still present underground.

After inerting the mine and extinguishing the coal fire, the next big challenge is overcoming the severe temperatures in the mine.

The NZ Fire Service and Rural Fire organisations plan to pump water up from a nearby creek to a holding tank above the charred ventilation shaft.

The water will then be pumped from the tank to the ventilation shaft and associated concrete pad, with temperatures on the pad at about 150 degrees.

The spokesperson said this effort was at a “beginning phase” and would be a long process because of the “very difficult terrain”.

“[The] only access is by helicopter basically,” he said.

Pumps, hoses and a water-holding tank will need to be airlifted to a safe location near the ventilation shaft.

Underground coal fighting specialist David Cliff told reporters on Friday that the mine recovery operation was very challenging.

"I've never been to a mine where we've had so many natural difficulties in terms of the terrain, the isolation and inability to get access to do things," he said.

Cliff is an associate professor and operations manager at the University of Queensland’s Minerals Industry Safety and Health Centre, and is very experienced in fighting fires in underground coal mines.

While the families of the 29 miners who died in the Pike River disaster want the bodies recovered, the severe temperatures underground will destroy forensic evidence and could leave little remains of the men.