Sage O'Donnell, a resident of the southeasterly state of Victoria, was allegedly killed in the days before Christmas, according to ABS.
In a statement issued by DFAT, O'Donnell's mother stated that her son "killed in action protecting the independence of the Ukrainian people."
Sage took action because he felt bad for the injustice being done to the Ukrainian people. He was astounded by the friendliness and kindness of his new neighbours in Ukraine, she said.
Although DFAT claimed to be "offering consular support to the family," it did not say where exactly O'Donnell was killed. Since Russia invaded Ukraine in February, at least four Australians have died there, including O'Donnell.
Over the last ten months, thousands of volunteers from all over the world have rushed to Ukraine to protest what they see to be Russia's unjustified invasion of the nation.
Since the fighting began in February, 1,956 "foreign mercenaries" have been killed in combat, according to a June report from the Russian defence ministry. According to Moscow at the time, around 1,700 foreign combatants invaded Ukraine to take part in the conflict, down from an initial estimate of up to 7,000.
Three foreign combatants were given death sentences by a Donetsk People's Republic court in the same month, a rebel state that Russia has occupied. There were a total of three men: a Moroccan and two Britons. The death sentence has received harsh criticism from the UN.