A missing radioactive capsule the size of a tic-tac has been in regional Western Australia, but the probe into how it became lost continues.

The capsule was found just two metres off the Great Northern Highway, south of Newman.

Dr Karl Kruszelnicki told Laurel, Gary & Mark it was like “finding a needle in a haystack, within a haystack with a needle in it”.

“How did they find it? It puts out gamma rays, so they would have had a gamma ray detector,” he said.

“If you drive really fast, you can cover a lot more ground but you’re not picking up that much dose because you’re going past it fast.

“It turned out that about 50 kilometres an hour was enough to pick it up!”

Press PLAY below to hear how dangerous the capsule could’ve been if found by a member of the public 

Image: WA DEFS