Eight years and six months after the Pike River Mine explosion, the first people re-entered the mine this morning.

Family members cheered from outside the mine as a recovery team broke through the seal of the mine and entered.

Sonya Rockhouse lost her 21-year-old son Ben in the November 2010 disaster, which also claimed the lives of 28 other men.

Speaking after the entrance was opened for the first time, Rockhouse said it "hit me in my heart" to stand there and watch the doors open.

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"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."

The Pike River Recovery Agency opened the doors and went inside and checked the drift's condition before heading back outside to a cheer from the families.

The group representing the families of the 29 men, the Pike River Family Reference Group, welcomed the unsealing of the mine held privately for the families.

The Pike River Mine Recovery team break through the seal at Pike River Mine. Photo / Supplied
The Pike River Mine Recovery team break through the seal at Pike River Mine. Photo / Supplied

Group member Anna Osborne, who lost her husband in the mine explosion, described the opening of the doors as "incredibly emotional".

"Watching those doors open and seeing the light enter that dark tunnel for the first time in years was incredibly emotional," she said.

"We've known we are going back in for a year now, today it feels like it. This is the start of a journey that will end with truth and justice."

A well-publicised re-entry attempt was planned for May 3 but called off by the Pike River Recovery Agency the day before for safety reasons.

The agency got an "unknown reading of oxygen" from a borehole 2.3km into the mine's drift, where the roof collapsed in the 2010 explosions.

The oxygen had the potential for a "spontaneous combustion event".

A Pike River Mine recovery worker inside the mine. Photo / FRG
A Pike River Mine recovery worker inside the mine. Photo / FRG

A leaky sampling tube was to blame for the oxygen spike and was replaced before work on cutting into the concrete seal resumed.

Families were disappointed by the delay but accepted safety must come first.

However, the renewed attempt took place this morning and Prime Minister Jacinda Ardern called the re-entry "symbolic".

"Re-entry to the drift is going to take a number of weeks and months," she said on Monday.

"It's a progressive piece of work as they move beyond, of course, the barrier created by the concrete which will be removed as I understand it in the coming days."

The Coalition Government and the Opposition have long fought over the viability of re-entering the mine's drift.

Families gather at the entrance to Pike River mine today. Photo / Supplied
Families gather at the entrance to Pike River mine today. Photo / Supplied

The Labour-New Zealand First coalition agreement said they would "commit to re-entry to Pike River", while the National Party has always said it was too dangerous to do so.

Last month National accepted advice re-entry could be done safely, taking politics off the table on the issue which had been a bitter political battleground.

Tracey Keane-Harvey, whose son Riki died in the mine disaster, said she would polish a piece of the mine's seal and "make a trophy and send it to National".

"It has been good finally to get to this stage and maybe, just maybe, one day one of us may be lucky and also to have some justice for all of our men," she said.

"It was hard them looking back and seeing all the lights, something we wished we could have seen nearly [10] years ago."

The Minister Responsible for Pike River Re-entry, Andrew Little, was proud to have fulfilled the promise of re-entering the mine.

first look inside inside the mine. Photo / FRG
first look inside inside the mine. Photo / FRG

He said New Zealand is not a country where 29 people could die at work without "real accountability".

"That is not who we are. And that is why today we have fulfilled our promise. Today we have returned," he said.

A Pike River Mine recovery workers broke through the seal for the first time today. Photo / FRG
A Pike River Mine recovery workers broke through the seal for the first time today. Photo / FRG

"The tragedy that took these men's lives was the consequence of corporate and regulatory failure.

"Fulfilling the promise to do everything possible to safely re-enter is an act of justice for families who have waited for far too long."

The appears to be a lot of groundwater in the mine. Photo / FRG
The appears to be a lot of groundwater in the mine. Photo / FRG