Federal police arrested a 27-year-old Victorian man in Brisbane on Thursday night, charging him with attempting to engage in foreign interference.
It's alleged the man provided the location and operation of Ukrainian military activities to Russian forces as part of the country's ongoing war.
AFP Commissioner Krissy Barrett said it will be alleged the man travelled to Russia in October 2024 for military-style training before returning to Australia.
"In Australia, it is alleged the man maintained contact with individuals he believed were connected to Russian intelligence services," she told reporters in Canberra on Friday.
"Investigators will allege the man then travelled to Ukraine in May 2025, where he joined the Ukrainian armed forces, gaining access to information relating to military personnel, units, and locations."
Police say the man then passed on information to Russia.
Ukraine has been at war since Russia's illegal invasion in 2022.
His arrest came following an investigation by the AFP's counter foreign interference task force.
It's the first time the task force has charged a Russian-linked person to foreign interference laws.
"Foreign interference is an insidious crime, and it has a bigger impact on Australia than what many people realise," Commissioner Barrett said.
"Offenders who engage in these crimes can leave our nation vulnerable to security threats."
The man will face Brisbane Magistrates Court on Friday.
Commissioner Barrett said police carried out search warrants at two properties in Brisbane on Thursday, and acted in relation to further warrants on Friday.