Brent Forrester reportedly worked at the mine from August 2008 to July 2009, well before the operation got to the milestone of starting hydro-mining in September 2010.

A friend of Riki Keane, one of the 29 men lost, Forrester told TVNZ’s Sunday program that the disaster should never have happened and was “100 per cent preventable”

Forrester worked during the development of the mine and shed light on his experience of the conditions around the continuous miners.

He told the program there weren’t any methane sensors in the area he was mining.

“If we had a gas problem, in the explosive range, the control probably wouldn't even read that [until] probably 15 or 20 minutes later," he told TVNZ.

Forrester also claimed the sensors were not always calibrated.

He revealed why the initial reports of the number of missing miners underground had varied so much.

"I believe some guys had forgotten to take their tags off the board. Things like that did happen and happened quite regularly," Forrester said on the television program.

"I don't believe there were accurate measures put in place to know exactly who was down there."

The former Pike miner said that safety issues were reported to Pike mine management, but “it didn't seem like anything ever really got done about it”

Forrester also referenced other reported evidence of ventilation and gas problems at Pike before the first tragic explosion.

"There's evidence of it, through different texts that miners have made, through comments they've made leading up to the explosion. There's been gas issues, there's been ventilation issues," he said.

Conrad Adams, a deputy at Pike and one of the 29 killed, reportedly sent text messages complaining about problems with the main ventilation fan more than six weeks before the first explosion.

According to the New Zealand Herald, Adams sent a “friend” the following text on October 6:

“Fan on stop 20 hours. Whole mine gassed out to f**k. Off to de-gas. Mining updip has its down sides ay?"

The newspaper also reported that another text sent on October 19 revealed the mine had gas issues in its development areas.

"Are you allowed to cut with a continuous miner updip in 4 cubic metres per sec from fan? Gassy area? Strong smell of hydrogen sulphide as well as from drillers stub," Adams reportedly wrote.

Whittall’s views

Pike River Coal chief executive Peter Whittall told ILN last week that the gas-monitoring systems at Pike were of the same standard as those used in mines in New South Wales and Queensland.

Whittall told TVNZ that the accident might have been preventable on the day but they wouldn’t know until the investigation was complete.

He noted that Forrester had not seen improvements in the ventilation shaft or the addition of auxiliary fans.

"We increased the air up to 80 to 90 cubic metres per second, which gave more than enough ventilation for those areas,” he told TVNZ.

“And then in recent months we've installed the first of our two main underground fans, and that increased it by another 30 per cent on top of that."

Whittall reportedly was unaware of issues around calibrating the gas sensors, and miners were aware of their responsibility to remove their tags from the board when they came to the surface.

Pike chairman John Dow confirmed to Radio New Zealand that workers could raise safety concerns with management and they would be acted on.