The identity of the soldier who killed the Red Baron, Germany's most celebrated World War I pilot, has long been the subject of controversy.

British, Canadian and Australian forces have all laid claim to the honour.

But as we commemorate the 100-year anniversary of the event, local historian Barry Butler and aviation expert Hawke Bear of Tyalgum on the north coast of NSW are convinced all evidence points to Sergeant Cedric Popkin.

"Out of all the different angles they've worked out that people were shooting from, Popkin was the only one who could have made that shot, with that 800-yard distance," Mr Bear said.

Mr Popkin was born in Sydney in 1890 and moved to the Tweed Valley on the north coast of NSW around 1907, where he was a jack of many trades — carpenter, postmaster and also a tobacconist.

He was 27 when the Red Baron was shot down over Morlancourt Ridge .

Hero to the German people

Manfred Albrecht Freiherr von Richthofen was a Prussian aristocrat. He originally joined the cavalry but transferred to the German air service in 1915.

Richthofen was an accomplished marksman and quickly earned a reputation as an ace fighter pilot.

He started painting his planes red.

"What that was about was so that he could be recognised in battle and so he would get accredited with any kills that he got," Mr Bear said.

As his reputation as the most daring of the German pilots grew Richthofen became known as the Red Baron and went on to lead the Jagdgeschwader 1, often called The Flying Circus or Richthofen's Circus.

He claimed to have won 80 air battles and was a hero to the German people.

German pilot Manfred von Richthofen became known as the Red Baron. ( Creative Commons: Photographer C.J. von Dühren. The Wartenberg Trust. )

The day the Red Baron was shot down

The western front in France saw some of the bloodiest battles during the war.

However, on the morning of April 21, 1918, it was quiet for the troops at Morlancourt Ridge near the Somme River.

"Cedric Popkin was a master-gunner in charge of four positions along the ridge", Mr Bear said.

"Just before the men gathered for lunch they saw a plane coming out of the clouds and flying towards them," Mr Bear said.

The plane was a Sopwith Camel fighter bi-plane of the British Royal Airforce.

"They realised that there was a German triplane on its tail and as it got closer they saw it was a red German triplane," Mr Bear said.

"[Popkin] took over the gun position."

The story handed down in the family is that Mr Popkin recognised the triplane as the Red Baron's and fired 80 rounds at it as it flew low across the beet field, chasing what he said was an inexperienced pilot.

Manfred von Richthofen painted his plane red so German troops would recognise his plane in battle. ( Creative Commons: photograph by Oliver Thiele )

"To hit the Red Baron's plane he actually had to aim at the Camel in front of it which is pretty unnerving really because you don't want to be shooting at your own guys," Mr Bear said.

A Canadian pilot in another Sopwith Camel joined the air chase and fired on the Red Baron from behind.

"The Red Baron broke all of his own rules by doing a stupid thing on this particular morning flying low across enemy lines," Mr Bear said.

'I am finished'

Mr Popkin calculated the Red Baron's return route would bring him just above the ridge line.

"He lined up his gun and fired another 70-80 rounds," Mr Bear said.

"The triplane pulled up in the air, almost stalled, turned to the right, almost rolled over on its right side and did a half turn, then crash landed into the beet field."

Richthofen got hit by one bullet through the right hand side of his ribs.

"It went through his body and popped out just near his left nipple," Mr Bear said.

He was still alive when the soldiers arrived at the wreckage and is reported to have said "alles kaput", which means "I am finished". Then he died.

The bullet was found in Richthofen's clothing when the allied forces examined the body.

The Red Baron was buried behind his enemy's lines and was given a full-military service by the Allied forces.

Controversy over who fired

Many eyewitnesses said that it was the Australian gunners in the trenches on the ridge that hit the plane and delivered the fatal shot.

However, at the autopsy conducted in the nearby field air base the RAF claimed it was the Canadian pilot Captain Arthur Roy Brown.

Another Australian soldier, Gunner Robert Buie, also claimed that he shot down the Red Baron.

Mr Buie was a fisherman from the NSW Central Coast.

"He's was on a different kind of machinegun, a Lewis gun and was shooting at the triplane head on," Mr Bear said.

"However the evidence points to the bullet coming from the ground, and from the side of the plane."

The Popkin and Buie families both still claim their family war heroes shot down the Red Baron, but Mr Bear and Mr Butler are certain that it was Popkin who did the deed.

Evidence points to Gunner Cedric Popkin having a good claim to the title of the soldier who shot down the Red Baron. ( Supplied: Popkin family archive. )

Commemorating Cedric Popkin

On a toilet block at Tyalgum there is a faded cartoon of Gunner Cedric Popkin shooting down the Red Baron.

Mr Bear said a more fitting tribute was overdue.

"Ced lost his right leg in the war", Mr Bear said. "Two months after he shot the Red Baron down he was hit by shrapnel in the right leg.

"It developed gangrene and he lost his entire leg, including the hip joint."

Mr Popkin returned to the Tweed Valley after the war and his work as a house builder.

"Can you imagine climbing ladders with all of your tools and walking on a roof with one leg," Mr Bear said.

"They a were a different breed of person in those days."

Mr Popkin died on January 26, 1968, after retiring and living out the rest of his life in quiet seclusion at Fingal Head.

Mr Bear's research includes the stories told to Yolanda, Mr Popkin's daughter.

"She just always knew that her Dad was the guy who shot down the Red Baron."

Mr Bear will pay tribute to Gunner Popkin in his speech at the Tyalgum 2018 ANZAC Day service.