Winston Peters says authorities are covering up the possibility of "a mass crime scene" at Pike River Mine.
Mr Peters said he will refuse to form a Government without the right to re-enter the mine, making changing the law to allow entry a "bottom line" for any coalition agreements with New Zealand First.
Mr Peters told The AM Show authorities "were obsessed with sealing [the mine] and consigning this issue to history."
"That has ramifications for a whole lot of people, starting with the police - how they handled it post the disaster, going back to the management of the mine. There has got to be someone accountable."
- Video: Inside the Pike River mine - what police never showed the families
- Pike River families say they were never shown video - despite police and PM's claims
Mr Peters said evidence is slowly emerging and there will be more showing manned entry "hundreds of metres" up the mine.
The New Zealand First leader said re-entry of the mine is "about the quality of this country as a democracy" and accused "the establishment [of] treating people with utter contempt."
Footage leaked to Newshub shows a robot accompanied by two men inside Pike River three months after the disaster. Families of the 29 men killed in the disaster say they had never seen the footage before.
The Government say re-entry of the mine has been assessed as too dangerous, and the Government has to abide by workplace safety laws.
Watch the video to see Winston Peters on The AM Show.