Volpato was reported in July to have allegedly tested positive for cocaine after he was pulled over for speeding in Sydney.
New South Wales police said on Wednesday that a second sample taken at the time had also tested positive.
They said a 22-year-old man - widely identified as Italy-based Volpato by local media - was fined AU$722 (€450).
His driving licence has also been suspended since July.
Volpato, an attacking midfielder for Serie A side Sassuolo, made three appearances at the World Cup for the Socceroos after switching his international allegiance from Italy to Australia shortly before the tournament.
He was born and raised in Sydney before moving overseas and represented Italy at various youth levels.