A four-man team has moved beyond the 170m seal in Pike River Mine, the first people to do so in eight years.
It is seen as a major development in the painstaking job of examining the 2.3km entry tunnel to the mine.
The aim is to look for clues as to why the mine exploded and recover any of the remains of the 29 men who died there in November 2010.
Pike River Recovery Agency chief operating officer Dinghy Pattinson was accompanied by three others, including a structural engineer, when they opened the door in the seal and briefly inspected the other side.
He told RNZ's Checkpoint they moved 15 metres beyond the seal, doing forensic work and inspecting the integrity of the roof and walls.
"It's just a releif for us all for the fact that we've absolutely commenced a full recovery of the tunnel.
"That was the last barrier or the last hurdle ... and to get through that and to know we're now getting through the tunnel is a really good feeling."
Pattinson said he would be celebrating the achievement today with a glass of coke.