By Melanie Burton
Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) is working with the Western Australian government to locate the capsule, it said in a statement.
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. The loss may have occurred up to two weeks ago.
Authorities are now grappling with the daunting task of searching along the truck's 1,400 kilometre (870 mile) journey from north of Newman - a small town in the remote Kimberley region - to a storage facility in the northeast suburbs of Perth - a distance longer than the length of Great Britain.
(Reporting by Melanie Burton in MelbourneEditing by Alasdair Pal)