Hong Hoang migrated to Australia in 1993 on a humanitarian visa and recently applied for a service pension.

Veterans of foreign militaries are eligible for an Australian pension if they can prove they served with an allied country in a war in which Australia was engaged and incurred danger from hostile forces of the enemy during that service.

But the Repatriation Commission in 2013 found Mr Hoang had not rendered qualifying service.

Mr Hoang then appealed the decision to the Administrative Appeals Tribunal, which found in his favour earlier this month.

During the hearing in Canberra, the tribunal heard Mr Hoang had been studying when he was conscripted into the South Vietnam defence force in October 1972 and served until the war's end in 1975.

He said he had been a cadet in early January 1973 when his company had been ordered to go to a village to distribute propaganda material in the lead-up to the Paris Peace Accords, which were signed later that same month.

But on the way, his platoon struck a mine and was ambushed by the Viet Cong – a common tactic used by the guerilla force.

The platoon returned fire and retreated to a nearby forest until reinforcements could arrive.

He said 10 cadets were injured in the blast, three seriously.

The Repatriation Commission rejected his application as Mr Hoang's immigration forms said he had been studying at university at the time of the incident.

The commission instead found that Mr Hoang did not enrol in the army until late 1973.

But tribunal senior member Professor Robin Creyke overturned the commission's decision.

Professor Creyke, in a judgment published this month, found Mr Hoang to be a credible witness and his story was consistent with events in South Vietnam at the time.

An expert on the conflict told the tribunal that while he could not vouch for Mr Hoang's story, he noted it was both plausible and internally consistent.

Professor Creyke found that the lead-up to the Paris Peace Accords on January 27, 1973 had been an intense period where both sides in the conflict tried to "establish control over as much geographical area as possible to strengthen their negotiating hand at the [negotiations]".

"In those circumstances it was also likely that all available troops, including those under training, would take part in the indoctrination campaigns – the Spywar project – and might also encounter dangerous situations in areas controlled by the opposing army during that process," the judgment said.

Professor Creyke said Mr Hoang had enlisted in the armed forces of the Republic of Vietnam and therefore had been an allied veteran who served with the forces of an allied country during a period of hostilities.

"He was ambushed by Viet Cong guerillas, surrounded, and for a time forced to hide and to return fire. Some of his platoon were injured," the judgment said.

"He was at risk of, or in peril of, harm and incurred danger from hostile forces of the enemy. Accordingly he has qualifying service.

"That means the decision under review is set aside and Mr Hoang is entitled to apply for a pension."