A lost radioactive capsule that sparked an unprecedented public health emergency is now being held at a secret location in Perth, as authorities unveil the team who found the tiny device in outback WA.

The minuscule and potentially hazardous capsule, which made international headlines when it was lost along a 1,400-kilometre journey between a mine site and the city, was found on Wednesday.

Authorities have remained tight-lipped on where the radioactive capsule was being stored in Perth since it arrived last night, citing security concerns.

But among the first to spot the device on the side of a remote highway after an intense, six-day search was Prashant Maharaj.

The radiological emergency expert from the Australian Nuclear Science and Technology Association (ANSTO) said tests of their specialised equipment gave them confidence the device would be found if it was out there, no matter its size.

"I think we were pretty confident that if it was on the side of the road, we would find it," Mr Maharaj said.

Eureka moment detectors 'all went off'

That eureka moment came when all four of the detectors his team were using started buzzing instantaneously while they were trawling the Great Northern Highway 74 kilometres south of Newman.

"We had all four detectors going off … all of them went off instantaneously," he said.

"We went about 200 metres forward, turned around and came back slowly. Radiation, in terms of dose rates, goes down as you move away but as you come closer it rises up again and you can see this on your detection system."

The team got to work, cordoning off an area of interest they were going to scour for the almost pea-sized capsule.

The team then had to assess the danger to make sure they could retrieve the item without any harmful human impact.

"You don't want to stay there, just minimise your exposure as much as you can," Mr Maharaj said.

"It can be a daunting task for people to see high dose rates [on the machine] but under controlled conditions having the right equipment on, you can get quite close … but I wouldn't recommend standing there longer than 10 minutes."

Careful recovery process

A mock recovery session was put into play to carefully move the capsule.

"We were noting down all the doses [of radiation] people were getting when they were coming out," Mr Maharaj said.

"We used a lead retrieval pot that was available from defence and we used a CV reach to pick it up and put it in the pot.

"We then moved the pot out of there into the vehicle and then initiated a contamination check of the area to make sure nothing had come out of the capsule itself."

Despite health officials' desperation to find the capsule, Mr Maharaj said it wasn't an immediate feeling of jubilation for his team.

"It was exciting but once we realised 'this was probably it', the adrenaline kicks in and you start thinking about all the processes you learnt over the years," he said.

Authorities said the capsule did not appear to have moved after it fell from the truck, indicating that no-one had handled the potentially deadly object, which experts have told the ABC emits radiation roughly equivalent to 17 chest x-rays an hour.

Reinforcements and false alarms

About 60 people were part of the multi-agency team in charge of finding the radioactive source, including Mr Maharaj's team from ANSTO, the Department of Fire and Emergency Services (DFES), and the Australian Radiation Protection and Nuclear Safety Agency (ARPANSA).

ARPANSA acting assistant director of emergency preparedness Chennell Allan and her team had to travel from across the country to help the search effort.

"We were testing and packing all of our equipment … booking our flights, and sorting out children and everything else at home that we're leaving behind," she said.

"In the radiation space, you never know what you're going to get. Because in Australia, we don't have that much that could go wrong."

DFES district officer Adrian Hamill said the team had to respond to reports of a silver capsule being found about 70km north-east of Perth early on Wednesday.

"Team Charlie were investigating a report of a find in Chittering, which was very plausible," he said.

Despite closely resembling the physical description of the missing radioactive source, it turned out not to be the caesium-137 the team had been searching for.

"It reinforced [to us] that we're in for a long haul, if this is what we're going to be investigating … that took two hours just to investigate that," Mr Hamill said.

Nervous wait for confirmation

Later that day, Mr Hamill received the call everyone had been waiting for.

"I can actually tell you the time … 11:13am I received the call," he said.

When Mr Hamill asked over the satellite phone "you found it?", the room that was buzzing and full of activity suddenly fell quiet.

The team on the ground had to confirm that the spike in radioactivity on their detection equipment was in fact the missing radioactive capsule.

"We were pretty confident, but we needed that serial number, and that took quite a while," Mr Hamill said.

At 2:55pm local time, the team received confirmation — the missing caesium-137 that had reached international headlines had been found.