Australia's nuclear safety agency has joined the hunt for a tiny radioactive capsule missing somewhere in the outback, sending a team with specialised car-mounted and portable detection equipment.

Authorities have now been on a week-long search for the capsule which is believed to have fallen from a truck that made a 1,400km journey in Western Australia. The loss has triggered a radiation alert for large parts of the vast state.

The capsule, part of a gauge used to measure the density of iron ore feed, had been entrusted by Rio Tinto Ltd to a specialist contractor to transport. Rio apologised yesterday for the loss, which happened sometime in the past two weeks.

The Australian Radiation Protection and Nuclear Safety Agency said it was working with the Western Australian government to locate the capsule. It added that the Australian Nuclear Science and Technology Organisation has also sent radiation services specialists as well as detection and imaging equipment.

The truck travelled from Rio's Gudai-Darri mine, north of Newman, a small town in the remote Kimberley region, to a storage facility in the suburbs of Perth - a distance longer than the length of Britain.

Rio Tinto apologised yesterday for the loss, which happened sometime in the past two weeks

State emergency officials have issued a fresh alert to motorists along Australia's longest highway to take care when approaching the search parties, as vehicles carrying the radiation detectors are travelling at slow speeds.

"It will take approximately five days to travel the original route, an estimated 1400km, with crews travelling north and south along Great Northern Highway," Department of Fire and Emergency Services Incident Controller Darryl Ray said in a statement.

The gauge was picked up from the mine site on 12 January. When it was unpacked for inspection on 25 January, the gauge was found broken apart, with one of four mounting bolts missing and screws from the gauge also gone.

Authorities suspect vibrations from the truck caused the screws and the bolt to come loose, and the capsule fell out of the package and then out of a gap in the truck.

The silver capsule, 6mm in diameter and 8mm long, contains Caesium-137 which emits radiation equal to 10 X-rays per hour.

People have been told to stay at least five metres away as exposure could cause radiation burns or radiation sickness, though driving past the capsule is believed to be relatively low risk, akin to taking an X-ray.