Deployment of the "GAG" unit at the Pike River coal mine has been delayed after a fire at the entrance to the mine this morning.

The "GAG" machine, a modified jet engine, will pump the mine with inert gases in an attempt to extinguish a coal fire and recover the bodies of the 29 men trapped inside 10 days ago.

Police said foam being used to seal the mine so that the machine can be used self-combusted about 6.30am.

They said the blaze was unrelated to the fire inside the mine and was safely extinguished.

Sealing of the mine was 90 per cent complete and was expected resume once the entrance had cooled, police said.

Once the mine has been sealed, the Gorniczy Agregat Gasniczy (GAG) engine will be used to pump non-volatile gases into the mine, displacing oxygen feeding the fire inside.

Pike River CEO Peter Whittall told Radio New Zealand this morning workers were up to the last stage of sealing the mine - putting in shotcrete or liquid concrete.

He said if the mine was not sealed by mid-morning, it would be this evening before the GAG unit could be started, as the heat of the day would reverse the ventilation into the mine and make it dangerous.

The unit would likely be started at 5pm unless another explosion burned off all the gases in the mine and allowed it to begin work earlier, Mr Whittall said.

"While that's [another explosion] terrible to think about, it actually may quicken our processes because that will burn off the gasses and get us back to a point of certainty and we can get in and start the GAG straight away."

Yesterday Mr Whittall said the fire in the mine - which had been burning gas since at least a week ago - had begun burning coal after a fourth explosion.

Flames were last night shooting from the top of a 108m ventilation shaft at the mine.

The fire is visible from a helicopter, belching smoke and flames from the wrecked top of a ventilation shaft which runs longer than a rugby field into the mountain. No flames were previously visible, even during the initial methane explosion.

Coal range

Mr Whittal said it was hoped that loose coal dislodged during the four explosions was burning.

He said that was preferable to the fire getting into the mine's walls of solid coal, which would create vast amounts of heat and be difficult to extinguish.

Mr Whittall likened this situation to a coal range.

"The worst-case scenario is that the walls of the mine, which are coal, can burn. That's a lot harder to smother because it just sits there like when you shut a fire in overnight in the old coal ranges.

"They just sit there and virtually do nothing, then in the morning you open up the air and they start again."

The length of time it took to kill the fire would indicate how much coal is burning and therefore whether the mine walls and the coal seam in the mountain are alight.

Authorities have previously said it could take several weeks before the fire went out even after it was starved of oxygen.

Grey District Mayor Tony Kokshoorn said there was "total silence" from the families of the lost miners when they heard the latest update.

"They're totally subdued and they're desperate now," he said. "How long do they have to wait to get their loved ones out of there?"

Strongman mine

The Strongman mine, where 19 men died in 1967, continues to burn near Pike River, giving off plumes of smoke after 43 years.

Mr Whittall said it was unlikely Pike River would burn indefinitely in a similar way because unlike other mines in the area, it did not have cracks to let fresh air in.

The Strongman disaster - like the one at Pike River - was caused by a methane explosion.

Two bodies from the Strongman mine were never recovered.

The section of the mine where the blast occurred was sealed off, but the mine was reopened.

Police Inspector Mark Harrison, who is leading the recovery operation at Pike River, said the fire made it less likely that the bodies of the 29 men could be recovered intact.

- With NZPA