It was in the shadow of the Gallipoli campaign that the Australian Red Cross first sent investigators to Egypt with the most difficult of detective tasks.

The "searchers", as they were called, were asked to learn the fate of thousands of Australian soldiers who were missing in action during World War I.

Flinders University Chair of History Melanie Oppenheimer, who is part of a new documentary telling the story of missing soldiers in the Great War, said chaos on the Gallipoli Peninsula prompted the Red Cross to act.

"You get something from the Army saying 'so and so has been hurt, more details following' or something like that, and the families are really distraught back home — they have no idea."

She said many families back in Australia struggled to understand how their loved ones could simply disappear while serving abroad.

"We now know that many bodies were never found, that they simply disappeared into the mud of the Western Front or indeed Gallipoli, but if you are a family member you don't understand that," Professor Oppenheimer said.

"What do you mean they are missing? What do you mean they don't have a body? How can the Army say they don't know where they are?"

The Red Cross London office of the Wounded and Missing Enquiry Bureau in 1918. ( Supplied: The Missing )

Enter the searchers

Led by Vera Deakin, the daughter of former Prime Minister Alfred Deakin, the searchers were all volunteers, many of whom were lawyers or men with some legal training.

They first set up office in Egypt and then travelled to London, with 32,000 cases sent to this team during the Great War.

Vera Deakin (centre) travelled to Egypt, where she eventually led the Red Cross's efforts to search for missing soldiers. ( Supplied: The Missing )

Professor Oppenheimer said the searchers would go through hospital wards trying to find a soldier who had served with or knew the missing person they were investigating.

"Eventually they will find someone who says, 'Yeah I know him, he was a mate of mine, he was right next to me, this is what I think happened'."

The searchers had a list of questions to ask witnesses, but they also were required to corroborate facts with two or three people before a report was sent back to a family, informing them of the fate of their lost soldier.

"There is not a lot of happy endings unfortunately," she said.

"Sometimes what the families are hoping for is mistaken identity."

Even though the news was rarely good, Professor Oppenheimer said it was still important for the families to know what had happened to their missing son or brother or husband.

The Red Cross searchers compiled detailed notes for the families of soldiers who went missing during WWI. ( Supplied: The Missing )

The soldiers who stayed behind

While the searchers worked tirelessly during the war, it was a different group that stayed behind after the fighting ended to help identify fallen soldiers.

It was only in the past decade that academic Fred Cahir discovered his grandfather, Frank Cahir, spent three years after the conflict taking photos of war graves for the families.

"I found it such an intriguing story about the 1,100 odd men who stayed behind after the First World War had finished and completed this gruesome and arduous task of identifying, locating and reburying the thousands and thousands of Australian soldiers who did not have a known grave," he said.

Dr Cahir said it was about 15 years ago that he discovered his grandfather had died by suicide.

"It had been kept a family secret for quite a few decades," he said.

Dr Cahir said his grandfather's life story, including his war history, had been lost to him in those decades because of his death.

He said he was astounded to learn about the work his grandfather and the members of the war graves unit did.

"There were occasions when they were literally digging up the remains of their mates. They knew all the places where Australians had died, they knew intimately its tragedies.

"But this was a new kind of horror for them.

"They had seen the horror of trench warfare but this was a new kind of horror."

More than 1,000 Australian men stayed behind after World War I ended, to help identify fallen soldiers and photograph their graves, for their families. ( Supplied: The Missing )

Dr Cahir said he thought of his grandfather's service with mixed emotions.

"I suppose we will never know whether the toll was too much from that work or not, but certainly we have so few records from those war grave workers that it kind of demonstrates it was a topic they didn't want to share."

The Missing documentary can be viewed online.