During the Vietnam War Derrill de Heer was responsible for "demoralising the enemy" through a chilling propaganda tape known as Wandering Souls.

As a member of the Australian Army's Psychological Operations Unit, Mr de Heer would broadcast recordings of voices — pretending to be the Vietnamese dead — across the battlefields at night.

"This voice would be going through the jungle and they would be hearing: 'Daddy, where are you? I'm wounded and I'm wandering'," Mr de Heer said.

"The idea was to demoralise the enemy, to get them to return back to the government under an amnesty program."

But, decades after the war ended, the Vietnam veteran has found himself on a new service mission — tracking down Vietnamese war dead still missing in action and helping bring the hundreds of thousands of "wandering souls" to rest.

Battlefield maps identify likely Vietnamese burial sites

Now 75, Mr de Heer says he has a sense of "unfinished business". ( ABC News: Adrienne Francis )

There were six Australian defence personnel missing in action after the Vietnam War but their bodies have since been repatriated to Australia.

But the remains of an estimated 300,000 Vietnamese killed in battle have never been found.

Mr de Heer felt Australia was obligated to help the Vietnamese track down their war dead by exchanging all battle data.

"The Vietnamese Government helped the Australian Government and the Australian military to find our six missing in action," Mr de Heer said.

"We didn't give them the information of where we killed people and buried them.

"[Australian] Defence says they don't have a legal obligation, but certainly there's a moral or ethical thing to do."

Mr de Heer eventually joined forces with military historian and Vietnam veteran Bob Hall and the pair started channelling their shared sense of "unfinished business" into a project coined Operation Wandering Souls — Bringing Them Home.

Assisted by a small team of dogged volunteers — including Mr de Heer's wife, Christine, who is a former librarian — they spent more than eight years digitising, proofreading and analysing declassified Australian war diaries containing battle descriptions.

The data was then visually transformed into detailed battlefield maps.

Mr de Heer's wife Christine is one of the volunteers who has worked for years correlating data for the digital battle maps. ( ABC News: Adrienne Francis )

Australians were involved in more than 6,000 combat incidents during the Vietnam War, and the battlefield maps show the likely burial sites of some 3,000 of the more than 4,000 Vietnamese killed by Australian soldiers.

"We are not looking for the graves ourselves … we are telling them where they are located," Mr de Heer said.

"We have given them references to places."

The mapping software will this week be demonstrated in Canberra to a large delegation of Vietnamese military personnel, headed by the Vietnamese deputy minister of defence.

"We believe that we can help them," Mr de Heer said.

An interactive map of Vietnam battle locations featuring data correlated by Mr de Heer and Dr Hall. ( Supplied: UNSW Canberra )

Diaries, letters, photos among hundreds of treasures returned

As part of Operation Wandering Souls, the veterans have also tracked down Vietnamese diaries, letters, photographs and artworks collected from the battlefield and brought back to Australia.

Some of the items were collected by Australian defence personnel for intelligence, while others were kept as personal souvenirs.

Over the past five years Mr de Heer and Dr Hall have made three trips to Vietnam to return more than 300 personal treasures.

Mr de Heer returns a portrait of a Vietnamese woman to her son. ( Supplied: Derrill de Heer )

One of the most rewarding returns was a large ink portrait of a woman that had been taken from a house in a burning, abandoned village.

With the help of the Vietnamese media, Mr de Heer was able to find the artist's family and deliver the drawing to the son of the woman in the portrait.

"The son said that when he went to the Vietnamese newspaper office and saw his mother, the smile never left his face," Mr de Heer recalled.

"It's very emotional stuff."

Mr de Heer urged anyone with Vietnam War artefacts or memorabilia to consider donating the items to Operation Wandering Souls.

"We have got Australian veterans who are getting old," Mr de Heer said.

"They will die and they will have stuff in their trunks.

"We worry that their children won't know what it is and throw it out and it will go to landfill."

Mr de Heer and Dr Hall return a painting, believed to be of a village patron saint, to a museum in Vung Tau. ( Supplied: Derrill de Heer )

Now 75, Mr de Heer has no plans of retiring from his all-consuming project.

He is now working through data from declassified American military records covering some 900,000 combat incidents.

In the near future Mr de Heer wants to return to Vietnam to teach local authorities how to use the mapping tools in their ongoing search for war dead — something he hopes will help "bring more wandering souls to rest".