Federal police have been working with Bulgaria to find an Australian suspected of being involved in a July bomb attack that killed five Israeli tourists.
The Bulgarian government said overnight on Tuesday it was seeking an Australian passport holder and a dual Canadian-Lebanese national.
It has made a "justified assumption" the pair belong to the military wing of the terrorist group Hezbollah.
"All Australians would be proud that the AFP has played an important role in this investigation to date."
Five Israeli tourists and their Bulgarian driver were killed in the bus bombing at Bulgaria's Black Sea Burgas airport.
It was the deadliest attack on Israelis abroad since 2004 and also killed the bomber and left 30 people wounded.
Bulgarian interior minister Tsvetan Tsvetanov said authorities had been tracking the two suspects.
Canadian foreign minister John Bair said his country was taking the alleged involvement of a Canadian passport holder very seriously.