Next Monday is the eighth anniversary of the tragedy at the mine. Two men survived but 29 died.
"Re-entry to the Pike River drift is an extraordinarily complex undertaking. This is a site which, even eight years after being the scene of multiple explosions and taking the lives of 29 men, poses major hazards. This has required incredibly robust planning," Little said.
The agency had recommended re-entering the drift via the existing entry. Cabinet this week approved an additional $14 million in funding, taking the total budget for the plan to $36m.
Little also considered the advice of independent adviser Rob Fyfe before making his decision on the options.
The Family Reference Group, which had its own experts, was also consulted throughout the process.
The first major task, breaching the 30m seal inside the drift, was likely to begin in February.
Little said much of the preparatory work had been done, such as making new emergency portal doors, restoring power and setting up a nitrogen plant to push out dangerous gases from the mine before oxygen is pumped in.
Little said there was always uncertainty in such projects but health and safety was paramount.
He quoted from the agency's report: "There is a lot we do not know and will not know until we are confronted with the situation as we find it underground. This will require agile thinking, the courage of all to say no if we are uncomfortable, the preparedness to reassess, reset and re-plan when necessary, and knowing when to call it 'quits'."
Little said that since the middle of this year, police had been actively involved in any re-entry plan so they could prepare for any forensic examination and victim identification.
He said the Government, and the three partners that made it up, committed to fulfilling the original promise made to the families, to do everything practicably possible to re-enter the drift and recover any remains and better understand the cause or causes or the original explosion on November 19, 2010.
Discussing the decision to go with the single-entry option, Little said safety was paramount. Ventilation was a major consideration.
The other two options, either drilling a bore hole or another tunnel, would also have required a huge number of helicopter movements to remove spoil from the digging.
Sonya Rockhouse, Anna Osborne and Bernie Monk, who all lost loved ones in the explosion and who have been fighting, alongside other families for re-entry, stood alongside Little holding photos of those they lost.
It is a truly amazing day for the families today.
Monk and Rockhouse both lost a son each, and Osborne lost her husband.
They were visibly emotional as Little announced his decision.
Osborne said the families fought "really hard" for this day.
"This is a victory for the little people of New Zealand."
She said 29 men went to work that day and no one expected they would never see them again.
"It is a truly amazing day for the families today."
She said there needed to be changes in workplace health and safety, with more than 400 workplace deaths since the mine tragedy.
She thanked the late unionist Helen Kelly "from the bottom of our hearts".
Pike River Recovery Agency head Dave Gawn said there was still a lot not known and there was still a lot of work ahead.
"Safety comes first, then operations, but we are confident we can recover the drift and get this operation completed."
Little said he was confident in the advice he had received.
"This hasn't been a difficult decision because of that."
He was confident of the operation's success.
He said there was a possibility there would be bodies inside the drift and the effort should be made to find out.
