An Australian man has been killed fighting in Ukraine, the Department of Foreign Affairs and Trade (DFAT) has confirmed.
The mother of Victorian man Sage O'Donnell said he "died in action defending the freedom of the Ukrainian people", according to DFAT.
He is believed to have died just before Christmas.
DFAT confirmed it was "providing consular assistance to the family of an Australian who died in Ukraine".
His mother said the family was "devastated" by the loss of Mr O'Donnell, whose "humour, kind heart, values and laughter will be dearly missed".
"Sage died in action defending the freedom of the Ukrainian people … [he] has always believed in defending country, people and the right for freedom," his mother, who has not been named, said.
"Sage passionately upheld his values and served here in his own country and most recently in Ukraine.
"Sage chose to take action based on his empathy for the Ukrainian people, and the injustice that is taking place.
"He fell in love with Ukraine and its culture and was humbled by the kindness and hospitality within his new community.
"Sage chose to live his life true to his beliefs and cause."
Irish journalist Norma Costello tweeted in August what appeared to be a video interview with Mr O'Donnell.
With much of his face covered by a dark green balaclava, Mr O'Donnell explained that he was hoping to use his military background to help train and protect volunteers fighting against the Russians.
"I'm able to come here and train them and provide another perspective that might save their life," he says in the video.
A Facebook page called "Memorial — International Legion Defense of Ukraine", which tracks foreign-born fighters in the war, paid tribute to Mr O'Donnell, offering "gratitude to our brother".
Mr O'Donnell is believed to be the fourth Australian to have died during the Russian invasion of Ukraine, which began in February.
Another Australian, Trevor Kjeldal, was killed last month in a mortar attack while fighting Russian forces in Ukraine's Donbas region.
'One of a kind'
Andrew Hewage met Mr O'Donnell when he travelled to Ukraine from Sri Lanka to help in the war effort.
He knew the Australian as "Don" and told the ABC they met at a training camp in western Ukraine in October.
"He was one of our colleagues," Mr Hewage said.
"He joined as a legionnaire, he was with us [on] the front line and later he joined as an instructor.
"He was a wonderful man … everything he did, being with us, the job he did, he was very fair in what he did and had a smile always on his face.
"There was no-one who was willing to say anything bad about him, and everyone was a good friend for him, and he was okay with everyone. Everyone. Every single person.
"He was a very humble guy, and he's one of a kind. He was a hero because he sacrificed his life for someone else, not for himself.
"Don was fighting a war which did not belong to him, but he fought that war for the better of the world. So everyone should know, people like Don should be remembered forever."
Mr Hewage learned of Mr O'Donnell's death from another colleague, and said he was shocked by the news.
"We did not expect this to happen," he said.
"Those locations were fortified, but unfortunately the shells have been hit directly to the location."
