Mining giant Rio Tinto apologised on Monday for losing a tiny but dangerously radioactive capsule that fell off a truck along a 1,400-kilometre-long stretch of road in Western Australia.

The solid, silver-coloured cylinder is smaller than a human fingernail – just 8mm by 6mm – but the authorities say it contains enough Caesium-137 to cause acute radiation sickness.

It disappeared last month from a truck that drove to the suburbs of Perth from a remote mine near the town of Newman, which lies 1,400 kilometres north of the state capital – further than the distance from Paris to Madrid.

"We recognise this is clearly very concerning and are sorry for the alarm it has caused in the Western Australian community," said Rio Tinto Iron Ore chief executive Simon Trott in a statement.

"We have launched our own investigation to understand how the capsule was lost in transit," he added.

The radioactive capsule, part of a gauge used in the mining industry to measure the density of iron ore, was transported by a certified Rio Tinto contractor, he said.

People should stay at least five metres away from the capsule, which emits beta and gamma rays with a radiation level equivalent to receiving 10 X-Rays every hour, the authorities have warned.

"If you have it long enough near you, it could cause what is known as acute radiation sickness," Western Australia's chief health officer Andrew Robertson said when the public were first alerted on Friday.

The container it was in collapsed because of the vibrations of the road trip, he said. The capsule apparently fell through the hole left by a bolt that was also lost.

The gauge was originally picked up on January 12 from the Gudai-Darri iron ore mine near Newman and delivered to the Perth suburb of Malaga on January 16, Rio Tinto says.

But the package was not opened until January 25 when the gauge was found "broken apart" with the radioactive capsule missing. State police were informed on the same day. (AFP)