Authorities have launched an investigation to determine how a tiny but potentially deadly radioactive capsule got lost as it was transported from a West Australian Rio Tinto mine to Perth.

An urgent search continues for the 8 by 6 millimetre unit that is believed to have fallen off the back of a truck on its 1400 kilometre journey from Newman to a depot in the Perth suburb of Malaga.

Emergency services say they are hampered in their efforts by a lack of equipment and have called on the Commonwealth and other states to provide more, including units that can be fitted to a vehicle.

The Department of Fire and Emergency Services has deployed teams with handheld radiation detection devices and metal detectors along 36km of the busy freight route.

Superintendent Darryl Ray said they were concentrating on populated areas north of Perth and strategic sites along the Great Northern Highway.

"What we're not doing is trying to find a tiny little device by eyesight" he told reporters on Saturday.

"We're using the radiation detectors to locate the gamma rays."

Authorities are also using the truck's GPS data to determine the exact route the driver took and where it stopped after it left the mine on or about January 10.

But there are concerns the solid capsule may have already become lodged in another vehicle's tyre and potentially be hundreds of kilometres away from the search area.

It is believed a screw worked loose inside the large lead-lined gauge it was contained in and the unit fell through a hole left by the missing fastener.

Rio Tinto said it contracted an expert radioactive materials handler to package the capsule and transport it "safely" to the depot and was not told it was missing until Wednesday.

Chief Health Officer Andrew Robertson defended the WA government's decision to wait two days to inform the public on Friday, saying the mine and depot had to be searched and excluded, and the route confirmed.

He said the capsule was packed in accordance with the radiation safety transport and regulations inside a box bolted onto a pallet.

"We believe the vibration of the truck may have impacted the integrity of the gauge, that it fell apart and the source actually came out of it," he said.

"It is unusual for a gauge to come apart like this one has."

An investigation will look at the handling of the gauge and capsule at the mine site, the transport route used and the procedures at the depot in Perth after it arrived on January 16.

Police have determined the incident to be an accident and no criminal charges are likely.

Authorities has also ruled out theft at the depot before the box was opened on Wednesday, saying there was anti-tampering tape on the box.

The small silver cylinder is a caesium 137 ceramic source commonly used in radiation gauges.

Dr Robertson previously said the unit emits the equivalent of having 10 X-rays in an hour and members of the public should stay at least five metres away.

Contact could result in skin damage, burns and radiation sickness, including impacts to the immune and the gastrointestinal systems.

Long-term exposure could also cause cancer, however, experts say the capsule cannot be weaponised.

"Our concern is someone will pick it up, not knowing what it is, think this is something interesting (and) keep it," Dr Robertson said.