Minister for Pike River Re-entry Andrew Little says he would be surprised if a missing cabinet door from the explosion isn't stored away safely somewhere.
Sonja Rockhouse, whose son Ben was among the 29 men killed in the mine blast, said a photograph of an electrical cabinet door that was blown 100m to the mine surface has just come to light but no one can say where the actual item has gone.
She said the situation was "bloody outrageous" and proof a proper investigation was needed.
The door from a fan control box was photographed nine days after the first explosion in 2010 but has not been seen since.
The lawyer who represented some Pike River families at the royal commission hearings said he had no recollection of the cabinet door.
Nigel Hampton said he did not recall that piece of evidence.
"I don't recall seeing photographs of [the door] during the lead-up to and through the Royal Commission hearings, which is slightly disturbing."
Mr Little said neither he nor the Pike River Recovery Agency knows where the door is.
"The agencies investigating the original explosion in 2010 were the police and what was then the Department of Labour, now WorkSafe. I'd be surprised if they don't have [it] stored somewhere [with] all of the exhibits and the material they collected at that time as part of their investigation,'' he said.
Mr Little said he became aware of the missing evidence last week and the agencies involved in the original investigation were notified but would still be in the process of hunting it down.
The Opposition agrees with Mr Little that there is probably a simple explanation for where the door has got to.
National's re-entry spokesperson Mark Mitchell said evidence was carefully preserved and the agencies involved in the investigation needed time to locate it.
"I don't know if it is missing, I mean we need to give the inquiry team and the police a chance to actually go back and, like I said, check back through that chain of evidence.
Everything is normally scrutiously labelled and stored so it may be stored somewhere, so they just need to have the time to sort that out."
Mr Little said the evidence could be very informative.
"From the information we know this component may well be very relevant to establishing the cause or potential cause of the explosion.
"The Royal Commission did a pretty good job of their best guess of a cause, but anything we can get now that gives us a better understanding would of course be quite helpful.''
Mr Little said equipment being shipped from Australia was due any week now and re-entry would happen in a few weeks.
Several ministers were expected to attend the re-entry when it takes place and Mr Little would visit next week for one final check.
Police said now was the time for anyone with information about the Pike River mine disaster to come forward.
Assistant commissioner investigations Richard Chambers said this was one of the most complex investigations undertaken by police in recent times.
Police said they were making inquiries about the door and would talk to those involved in the original police investigation, which finished in 2013.
"We are aware of commentary about this item of equipment. Police are however unable to speculate about the relevance of this or any other item until we have completed our enquiries and have a full understanding of its significance or otherwise," he said.
"We note however that WorkSafe has today made specific comment on this item of equipment. It is important that police hold all relevant information to inform future decisions. We are therefore appealing to anyone with information which they believe is important and has not been considered by police to come forward.
"Our commitment is clear - we will do all we can to provide answers to the Pike River families in an open and transparent way."