The Pike River families say at least six key pieces of evidence, collected from around a ventilation shaft after the blast, are unaccounted for.
Newshub has obtained leaked photos showing evidence scattered around the Pike River Mine ventilation shaft.
Leaked photos taken less than 12 hours after the first explosion in the middle of the night show lumps of coal, mixed in with unidentified objects blasted up and out of the mine.
And that includes items that are clearly metal objects.
"They could potentially tell us the causation of the first explosion," said Dean Dunbar, who lost his 17-year-old son Joseph in the disaster.
The Pike River families are demanding to know what happened to all the evidence gathered from the scene.
"There is absolute fragments, coal, aluminium, etc, scattered all around that vent shaft," Mr Dunbar says.
So far one item, a piece of an electrical cabinet, blown to the surface from inside the mine's workings below, has been confirmed as missing by police.
Now Dean Dunbar has told Newshub he has seen an internal police document that shows that there are at least six items of interest blown out of the vent shaft, photographed and retrieved that were not handed over to police three months after the blast.
"The police want to know where they are, the police also want to know where the pictures are," Mr Dunbar says.
Notes obtained under the Official Information Act from an internal police debrief dated April 26, 2012, confirm many items are lost.
Police admit in the document that "exhibits arrived at investigation base with no documentation" and officers "did not know when or where they had been taken".
They stated the "chain of evidence for documentary exhibits including videos and photographs" was quote "diabolical". They also admit that "cameras disappeared that were bought solely for recording", with the record showing police "do not know where they went."
Mr Dunbar wants to know from police where the six missing items of evidence are.
"Look, if it's not a cover-up and it was negligence, then so be it. If they were under-staffed and under-resourced and lost, then so be it. Just tell the truth."
Police Assistant Commissioner Investigations Richard Chambers says police are making enquiries surrounding speculation the electrical cabinet is missing.
"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," Assistant Commissioner Chambers says.
"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."