The radioactive capsule was part of a gauge used to measure the density of iron ore feed from Rio Tinto's Gudai-Darri mine in the state's remote Kimberley region.

The capsule, believed to have fallen from a truck, was part of a gauge used to measure the density of iron ore feed which had been entrusted to a specialist contractor to transport.

It is unclear how long the radioactive capsule, part of a gauge used to measure the density of iron ore feed, has been missing.