An Australian expert was arriving in Greymouth tonight to provide advice on the jet engine GAG operation at the ill-fated Pike River Coal mine, where chief executive Peter Whittall says it's "highly likely" part of a coal seam has caught fire.

The fire in the mine was definitely coal, he told a media briefing this afternoon.

"It's quite highly likely it's a coal seam but that's not necessarily throughout the whole mine," he said.

"It's probably just around the bottom area, probably quite close to the seat of the fire."

He said it could be a combination of loose coal and part of the seam near the bottom of the shaft.

The mine was rocked by an explosion on November 19, trapping 29 men. Any hopes they had survived were dashed by a second large explosion last Wednesday and since then there have been two more blasts.

"The men would be potentially...500 metres away from the fire...," Mr Whittall said.

"There's hardly any ventilation so I would imagine the air temperature down there would continue to rise with the fire and it would be like a fire in one part of your house (which would) eventually heat the whole part of the house," he said.

If the coal seam was on fire, it would make the process of pumping inert gases into the mine much more difficult and the GAG jet machine that does this may have to be used several times.

Superintendent Gary Knowles, the police commander of the recovery team, said tonight bringing the expert to New Zealand was "crucial" in terms of all the emergency services getting around the table, "getting a better understanding of what we are going to face underground".

"We've been planning for some time the GAG operation and bringing the expert," Mr Knowles said.

"He can tell us what the options will be. What the risks are, and I think the primary focus has never changed that we need to stabilise that environment. We need to ensure that it is safe and poses no risks to anyone else."

Mr Knowles stressed the GAG operation was only one phase of the recovery operation.

"There are multiple phases to be looked at and explored. And as I keep saying we are ensuring that the families know what we are looking at before we go forward."

Mr Whittall said a separate fire, at the portal of the mine, had been extinguished but it had put back several hours of work and they would not be able to get the GAG machine running until midnight or 1am.

Once running, the GAG would take about four to eight hours to fill the mine and it may need to be used several times to complete the job.

The jet engine would blow through a 40-foot shipping container and another container would be on top to seal the mine as a "void filler" and would be covered with liquid concrete instead of polyurethane foam.

When asked if the bodies could still be intact, Mr Whittall said he couldn't speculate on that as he was not a forensic expert.

He said he was sure some families hoped "in their hearts" that there could still be someone tapping on a pipe, while others were totally resigned to the "total loss of life" and the possibility they would not get their loved ones out of the mine intact.

He would not speculate on the future of the mine.