The Pike River Mine relied on a gas sensor that was in a "sad state of repair'', the Royal Commission of Inquiry into the disaster heard today.
The commission heard yesterday the devastating explosion of November 19, 2010 could have been the result of a collapse in the area already mined that sent explosive gases racing through the mine.
That may have happened as the water pumps were turned on, re-powering the electrical system, which may have begun sparking and arcing.
Australian David Reece, one of five experts used by the Department of Labour to reach those conclusions, said today that a gas monitor in a panel had been poisoned by high levels of gas and did not work or had been disconnected.
"For even a moderately gassy mine to rely on one sensor at the top of the shaft, which was difficult to access ... is hard to comprehend.''
The sensor was in a "fairly sad state of repair'', he said.
Methane levels measured in the main shaft on November 15 and 17 were 2.75 per cent, but closer to the coalface they would have been at least twice as high.
However records from those days did not record this.
Mr Reece said it was apparent sensors were not being calibrated and maintained because two that were close together gave different readings.
He said Pike River staff reports and modelling after the disaster showed a "serious lack of ventilation'' for the number of coalfaces being worked.
"You could see that deputies at times were having trouble just getting air to move in the direction it should.''
Putting the main fan underground and in a high hazard area was "highly unusual''.
'Stoppings' in some parts of the mine, used to direct air around the tunnels, were simply "pogo sticks and brattice and plasterboard''. Another crucial one had a cloth trapdoor.
The pipeline meant to drain gas could not cope with the volume and was probably blocked.
Mr Reece said he thought the first explosion was probably fairly deep inside the mine.
The two survivors did not recall feeling a heatwave and the temperature at the point of ignition would have been 700 degress Celsius.
Mr Reece said in Australia there was a push towards three entries into a mine. Pike River had just one, plus a ventilation shaft.