Families of the Pike River Mine victims have described the "incredible" moment the concrete seal was broken.
They gathered at the gates of the mine to watch re-entry on Tuesday, as a loader brought into the mine removed the seal.
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- Pike River re-entry 'beginning of the end of a journey' - Sonya Rockhouse
The emotional family members released yellow balloons to celebrate the historic milestone.
Widow Anna Osborne said it was an intense moment.
"Watching those doors open and seeing the light enter that dark tunnel for the first time in years was incredibly emotional.
"We've known we are going back in for a year now, today [Tuesday] it feels like it.
"This is the start of a journey that will end with truth and justice."
The next step will be the entry of a recovery crew, which will search for any evidence as to what happened on that day in 2010.
"We've said for years that the drift could be recovered safely and now it is being recovered," said Rowdy Durbridge, whose son Dan died in the explosion. "I've walked that tunnel many times myself, but this time it'll be different.
"Right from the start there's been a lot of talk about the respect for the 29, I reckon that going in, getting the evidence, and finding them justice is the best respect we can offer."
Spokesperson Sonya Rockhouse said in a statement it was just one moment of a long process.
"To stand there today [Tuesday] and see the door open hit me in the heart.
"And to think, all this might never have happened if we hadn't blockaded the sealing of the drift and if New Zealand hadn't stood with us.
"It's just incredible."
Minister responsible for Pike River re-entry Andrew Little said the Government had fulfilled its promise.
"New Zealand is not a country where 29 people can die at work without real accountability.
"Fulfilling the promise to do everything possible to safely re-enter is an act of justice for families who have waited far too long.
"There is still much to do. We must find out what happened at Pike River."
Little said Tuesday's milestone belongs to the families' and the memory of the miners.
The seal was used to prevent access into the mine ever since the fatal explosions that claimed the lives of 29 men in November 2010.