Work to flush explosive gases out of the Pike River mine tunnel is set to begin today as the operation to re-enter the mine drift enters a critical phase.

Photo: RNZ / Tim Brown

The Pike River Recovery Agency said it was making good progress at the West Coast mine, where 29 men were killed in explosions in 2010.

Chief operating officer Dinghy Pattinson said a trial pumping nitrogen into 180m of the tunnel before Christmas was a success, reducing methane levels from 96 percent to less than 2 percent.

Work would now begin on purging the full tunnel, which was about 2km long.

That meant pumping nitrogen in every day for 12 hours a day. It was expected to take about two weeks to complete, Mr Pattinson said.

Morale was strong at the mine site, with some key jobs being ticked off, like installing emergency doors and starting to run nitrogen lines through the dense bush, he said.

The agency's chief executive, Dave Gawn, said those working at the mine were also starting to receive basic forensic training this week.

"It's forensics 101 - awareness training - so that anyone who enters the mine knows what to look for, knows how to manage and handle items of interest in accordance with police requirements."

Photo: Supplied

Family members, like Anna Osborne, who lost her husband Milton in the disaster, are hoping evidence may be uncovered that will lead to a prosecution.

"We owe it to the rest of New Zealand to keep fighting this fight because you don't allow 29 men to die at work and people to work away from that and wash their hands of it," Mrs Osborne said.

"There has been no justice. Our men deserve that and our families deserve the closure."

Like many family members, she has been a regular visitor to the agency offices in downtown Greymouth.

They are invited to sit in on planning and logistical meetings and have a weekly progress briefing.

Bernie Monk, whose son Michael died, said while it was great to see progress being made, he had always known the mine could be re-entered and questions why it had not happened years ago.

"It's been a very, very frustrating era for the families," he said.

"My heart opens to the families because they've been very patient. They knew this could be done and they persevered and the writing is on the wall now."

The agency said though it was "event driven" rather than timetable driven, if the work continued to go well people may be able to enter the mine between the end of February and the middle of March.