A New Zealand Mines Rescue team are to enter the Pike River mine at 10am this morning, the first time the mine has been entered since a blast tore through the mine on November 19 last year trapping 29 men.
A team of six will enter the mine through the sea container and concrete air lock at the mine's entrance, carry out a reconnaissance about 200m inside, and then select the best place to build a temporary air seal.
Gas monitoring equipment will be set up inside the mine by a second team.
The gases will be drawn out by a ventilation fan. The air lock at the front entrance will then be removed and permanent ventilation doors will be installed.
Rescue workers can then work their way into the mine stage by stage, installing further seals as they go to keep the environment safe.
"Everybody at the mine and involved in the process is very keen to be getting on with it," statutory mine manager Steve Ellis said.
Oxygen was at 2 per cent in the mine, and carbon monoxide was about 300 parts per million today.
"If there is higher oxygen and higher carbon monoxide and other indicator gases then we wouldn't be going anywhere because that would be an indication of accelerated oxidation, Mr Ellis told NZPA.
Mr Ellis said the spot 100m into the mineshaft - about 6m high and 6m wide - was the best place to put a seal - a pumped, cementitious grout, like expanding foam.
Once the seal was in place, which, if all going well, could take until the end of July, the gases in the mine would be monitored for about two weeks to see if the seal was working, Mr Ellis said.
Eventually a steel door would be installed in the seal and the plan was then to remove the old seal at the mine entrance, a shipping container embedded in concrete installed just after the explosions.
Mr Ellis said that next week feasibility studies would determine the logistics of a staged re-entry, carried out in similar stages. However, he did not know how many stages it would take to gain entry to the parts of the mine where the dead men's bodies are believed to be.
Meanwhile families of the 29 workers who died in the mine are relieved the mine is being re-entered at long last in an effort to recover the remains of their loved ones.
It is more than seven months since explosions in the West Coast mine left the 29 men dead, and experts are finally confident the gas levels are safe enough to go in today.
Images captured from cameras put inside the mine have suggested there may be bodies still intact inside.
"We are just really happy that finally it's started," said Neville Rockhouse, whose son Ben died in the mine. His other son Daniel got out alive.
"There is a sense of 'at long last'. It's been seven months and it's been rather arduous and very stressful ... and finally things are moving in the right direction."
Bernie Monk, who lost his son Michael, said he felt the re-entry should have begun months ago.
"But anyway, it's getting done now and I'm not going to argue about it. Everyone is now working together."
There has been speculation the operation to try to reach the remains could take up to two years.
Mr Ellis conceded it would take months.
But Mr Rockhouse said putting timeframes on the operation was guesswork.
It is still unclear who will pay for the costly operation to recover any remains, with the victims' families and their legal team looking to the Government after receivers took over at the mine and had no body recovery plan.
Prime Minister John Key has not ruled out giving taxpayer money to the venture, but says it would need to be a credible plan.
Mr Monk said the receivers, PricewaterhouseCoopers, want to prepare the mine for sale, and had committed to paying for re-entry until a rockfall 2.4km into the mine.