The Royal Commission on the Pike River coal mine tragedy has been told of a series of sudden rises in the level of methane in the weeks before the November explosions.
The commission is conducting phase three of its inquiry into how 29 men died in the West Coast mine almost a year ago.
The hearing has been told that up to a fortnight before the first explosion on 19 November last year the methane spiked to 1.5% to 2.5% of the air inside the mine.
A Department of Labour mines inspector Kevin Poynter said he was very concerned about that.
He says every one of the methane spikes should have been notified.
Mr Poynter also conceeded that the recorded levels could have been exceeded elsewhere in the mine.
The hearing heard that daily inspections three weeks before the explosion revealed the methane sensor was out of date.
While it was still working to record the spikes, it should have been checked but had not been since it was recalibrated a month earlier, Mr Poynter said.
He talked of "high potential incidents" which he said are dealt with promptly in Australia, where he went to work after leaving the Labour Department.
Mr Poynter says in hindsight the same should have been done in this country.
Full picture lacking
Earlier on Wednesday Mr Poynter told the hearing he could not get enough information to know for certain whether the mine was fully complying with health and safety regulations when he inspected it last year.
He said he made seven inspections between the start of last year and the time the mine exploded.
In total that amounted to between 21 and 28 hours of inspection.
Mr Poynter says what he got was a series of snapshots that did not give him a full enough picture.
He has previously told the commission there was often not enough time or resources to do the work expected, nor was there always sufficient oversight of that work.