The Pike River families have "full faith" Winston Peters will secure a manned re-entry into the mine, seven years after 29 men lost their lives.
Mr Peters is meeting with the families on Thursday morning, before he begins coalition talks with Bill English and Jacinda Ardern later in the day.
Families spokesman Bernie Monk told media in Wellington they knew they had Mr Peters' support.
"He's a man of his word, and we're going to support him the whole way… We've got full faith in Winston Peters and what he's going to do."
Earlier, Mr Peters declined to say what he will tell Mr Monk and the families, saying it was "confidential".
Asked if reentering the mine was still one of NZ First's bottom lines, Mr Peters responded: "What do you think? How many times do I have to tell you?"
He said the party wanted to do "what's right by those people".
Labour has already agreed to re-enter the mine, National hasn't.
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Newshub political editor Patrick Gower told The AM Show going into the mine was now not a matter of if, but when.
"The National Government tried to put concrete down that mine... we will now see the character of Bill English and all the other people in the Beehive who tried to stop [going back into the mine]. They've probably, already in their minds, agreed to letting men go back into the Pike River drift.
"They will not go into Opposition over this... They will literally go back on everything they've said in order to get Winston into power. They'll go back into Pike River... It's hugely embarrassing."
Mr Peters will meet Mr English in the morning, before talks with Ms Ardern and Labour begin in the afternoon.
Labour, United Future, The Māori Party, and the Greens signed an agreement with the families in August to re-enter the mine.
Explosions in the mine killed 29 men in 2010. Since then robots have been in, but not people. National is working on another unmanned entry by the end of the year, before committing to any future manned entries.
Sonya Rockhouse, who lost her son Ben in the Pike River disaster, said the election result - that put Mr Peters in a position to choose the next Government - was great news.
"Thanks to NZ First and all of the other parties who have backed our cause we will now finally get truth and justice," she said last month.
"We'll be making sure that any support Winston and other leaders need to get this done will be delivered."