Solid Energy chairman Andy Coupe said in February he would resign sooner than let anyone enter the unsafe mine.
However, most of the families have been pushing for a manned re-entry of the drift, and they were recently in Wellington talking to politicians.
The agency would take ownership of the mine and reclaim the drift to recover any remains and evidence.
The commitment was to be signed this afternoon by the leaders of Labour, United Future, Maori Party, Green Party and New Zealand First. That means there is now a parliamentary majority.
Anna Osborne, whose husband Milton died in the mine, said they were "incredibly thankful" to all of the parties that hd backed them.
"Their commitment shows that we can get justice, and lets all New Zealanders know that our leaders will do the right thing," Mrs Osborne said.
Bernie Monk said it was a "moment that we will remember for the rest of our lives".
"A moment when right won out."
The disaster is now approaching its seventh anniversary.
The commitment:
"We, the undersigned, commit that a new government we are part of will act immediately to safely re-enter, fully recover, make safe and comprehensively investigate the Pike River Mine drift. We will immediately create within the new government a minister responsible for Pike River re-entry. Within the first 100 days of a new government, we will create an agency for Pike River re-entry to take ownership of the Pike River Mine site and associated rights along with appropriate funding to plan and implement the mission."
- Greymouth Star
