A 13-metre-deep sinkhole opened up at a West Australian mine site, swallowing two unoccupied mining vehicles not far from where four people were working, a report has revealed.

The Department of Mines, Industry Regulation and Safety has released a "significant incident report" on the pit-floor collapse, which occurred last October.

The document did not disclose the name or location of the mine site, but said the area of the pit that collapsed had previously been mined, then backfilled in stages since 2016.

It also said surface drilling and blasting activities were taking place in the area when the sinkhole opened up, resulting "in the loss of an integrated tool carrier and an explosives truck that were parked in the blast pattern".

"A number of charged blast holes were also engulfed in the sinkhole," the report noted.

Hazard warning not heeded

It also revealed the area had been identified as a hazard in December 2017, but "recommended controls" had not been followed.

"At the time of subsidence, the backfilled underground workings were not treated as void, allowing work to be undertaken in an area of unknown stability," it said.

But it added that the exact cause had not been determined because of a lack of access, records and uncertainty about the "void shape and material".

The department's investigation into the incident is ongoing.