Experts have suggested a split-second gas outburst in the methane-heavy Pike River mine could have overwhelmed miners before safety alarms could respond.

The possibility of a sudden gush of methane from a coal seam, simultaneously ignited by machinery, is likely to form a key part of the inquiries into the explosion that tore through the 2.8km tunnel and led to the death of the 29 workers.

A third, smaller explosion shook the mine yesterday afternoon, but did not cause injuries or hinder the recovery operation.

Every detail of the mine's monitoring systems and the miners' movements on November 19 will be scrutinised to explain what experts have said is a "puzzling" and "surprising" disaster for a modern mine.

Pike River has this week been questioned as to why its safety devices did not warn miners of an explosive atmosphere in the tunnel.

Chief executive Peter Whittall says remote and hand-held methane-detection devices were operating on the day of the blast.

If methane levels reach 1.25 per cent, machinery is switched off in a mine. If it reaches 2 per cent, a mine is evacuated. At 5 per cent, methane combined with oxygen becomes potentially explosive.

Mr Whittall said the most recent inspections of gas levels had not given managers cause for concern.

A slow-build up of methane before the explosion was unlikely, because it would have triggered gas detectors.

Mining explosion and gas analysis expert David Cliff, who advised New Zealand emergency services during the vigil after the initial explosion, said a sudden gas outburst was a "credible" line of inquiry for investigators.

"The gas content in the coal seam at Pike River was close to ... potential for a gas outburst. When the methane content or gas content is very high and the drainage doesn't work or isn't adequate, then when you start to mine towards it, it can be suddenly released.

"I doubt the methane detectors would react quick enough to cut the power in terms of an outburst.

"They're intended to pick up a rising level, but an outburst can occur in a few milliseconds - too quick for the electricity to be turned off."

Mr Cliff said he would have expected alarm bells in the control room straight after the blast.

But the alarm was raised by miner Daniel Rockhouse, who had to recover from inhaling carbon monoxide before calling the base manager.

Engineering and mining geologist David Bell also said if Pike River's alarm systems were robust, a sudden break-in of gas would be a likely cause of an incident of this scale.

Legislation required gas monitors on all machinery and cutterpicks with cut-outs at certain levels to prevent sparks igniting gases.

But unlike in overseas mines, New Zealand miners are not required by law to drain methane as they drill.

Mr Bell said parts of the coal seam near the Hawera fault had shown more than 8m3 of methane per tonne of coal. At this concentration, Australian miners are required to drain the methane before continuing to mine.

Mr Bell is writing to Energy Minister Gerry Brownlee to strongly recommend a similar NZ requirement.

Yesterday, relatives took some of the cars and possessions of the dead miners from the mountain, while the search for their bodies continued.

A GAG jet engine has arrived in New Zealand. It may be used to blow inert gases into the tunnel, so any heat sources will not cause further blasts.

It is expected to be in use by Monday. It is capable of producing 7m3 of gas a second, so would take at least a day to fill the mine.

- Additional reporting: Hayden Donnell