Pike River families are shocked a potentially vital piece of evidence into the deadly explosion has gone missing.

An electrical cabinet that was blown to the mine surface has not been seen since it was taken by a helicopter leased by the company.

Former mines inspector Tony Forster, who's now advising the families of those killed in the tragedy, said it was flown to the Pike River Mine office.

"It blows my mind that something as significant as that, in an area that the royal commission centred on, has gone missing," he told TVNZ.

Bernie Monk lost his son in the explosion, and says it's unacceptable.

"It's coming up nine years, and we're still uncovering things that should have been done right in the first place. They've had this evidence, and we're asking them, where is it?"

Mr Monk says the cabinet could hold the answers to why the explosion happened on November 19, 2010, killing 29 men.

"We've known this has been a cover-up for a long time, and that's why we've fought so hard to bring things out into the open. It makes me angry that the families have had to do this," said Mr Monk.

"We're asking the authorities to go away and bring all this back to us and make sure that the truth is told to the country. The country deserves to know the truth of what's happened at Pike River."

Police said they became aware of the missing cabinet last week and are making enquiries, Stuff reported.

"Police would like to hear from anyone with information which could assist us on this, or any other issue which may be relevant to the original investigation," said assistant commissioner Tusha Penny.

An operation to re-enter the mine is expected to get underway at the end of the month, led by Mr Forster.

Newshub.