The 8mm by 6mm item fell out of a density gauge while being trucked from a Rio Tinto mine in the Pilbara to Perth.

Emergency services are searching a 1400km route amid warnings the Caesium-137 in the capsule could cause radiation burns or sickness if handled and potentially dangerous levels of radiation with prolonged exposure.

The Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) on Tuesday said it had sent a deployment team with specialised car-mounted and portable detection equipment to join the search.

Led by WA's Department of Fire and Emergency Services, the hunt is expected to take five days with vehicles travelling at 50km/h.

Radiation services specialists and detection and imaging equipment are also being sent to WA by the Australian Nuclear Science and Technology Organisation.

Rio Tinto has apologised and ordered an investigation into what went wrong during the haul, which was carried out by a contractor.

Emergency Services Minister Stephen Dawson flagged the WA government was likely to also probe the incident.

"How these are transported does need to be looked at," he told ABC radio.

"It does puzzle me how such a thing can fall off the back of a truck."

Rio said a bolt that secured the capsule within the gauge appeared to have sheared off, creating a hole just big enough for the item to escape.

The truck arrived in the Perth suburb of Malaga on January 16 but it wasn't until nine days later that a technician realised the capsule was missing.

The capsule is smaller than a 10 cent coin but the amount of radiation it emits is equivalent to receiving 10 X-rays in an hour.

Drivers have been warned it could have become lodged in their car's tyres.

Australian Associated Press