In videos circulated on social media, the visibly bruised and shaken man identifies himself as Oscar Jenkins, 32, as he is interrogated and beaten by his Russian-speaking captor.
"Live in Australia and Ukraine. I'm a teacher," he said, mixing broken Russian and English.
Standing in army fatigues in a forest, Mr Jenkins tells the man he wanted to help Ukraine and had previously lived in China.
According to a LinkedIn profile matching Mr Jenkins' details, he attended Melbourne Grammar School before working for seven years as a university lecturer in Tianjin, China.
The Australian government said it was making urgent inquiries into the reports.
Prime Minister Anthony Albanese says the government is investigating the man's alleged capture. (Mick Tsikas/AAP PHOTOS)
His capture was "concerning news", Prime Minister Anthony Albanese said.
"We're working through the Department of Foreign Affairs and Trade to provide support, including for this gentleman, trying to ascertain the details and the facts," he told reporters on Monday morning.
"We know that the Russians often put out information that isn't right so our embassy in Moscow is working.
"But in addition to that, Foreign Affairs and Trade are working here."
If the details are confirmed, Mr Jenkins would be the first Australian combatant captured by Russian forces in Ukraine.
Glenn Kolomeitz, a former Australian Army officer and lawyer, estimated upwards of 30 Australians were in the combat zone fighting with the Ukrainian foreign legion.
"I now suspect that this guy will be exploited for propaganda purposes, to apply some sort of leverage to the Australian government," he told ABC Radio.
Glenn Kolomeitz says at least 30 Australians are believed to be fighting with Ukrainian forces. (Mick Tsikas/AAP PHOTOS)
Foreign hostages were valuable for Russia as bargaining chips to put pressure on foreign governments supporting the Ukrainian war effort or as propaganda tools, Dr Kolomeitz said.
Russia has said foreign citizens fighting for Ukraine will be prosecuted as mercenaries and face up to 15 years in jail, which would contravene international law.
"They're not mercenaries," Dr Kolomeitz said.
"They are lawful combatants engaged in international armed conflict, and they're therefore entitled to full prisoner-of-war protections."
Mr Albanese said the government would make "appropriate representations" on behalf of Mr Jenkins.
"We always look after Australians," he said.
"That's the job of an Australian government - it is to make representations for Australian citizens."
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