He first thought he won $23.3 million (more than $16 million in US dollars), but then discovered he doubled his winnings because he mistakenly bought a second ticket for the same drawing using the same numbers.
He thought he had bought a ticket for a different drawing.
Now, the unidentified man will be $46.6 million richer ($33.15 million in US dollars).
Unlike the American lottery, the Australian lottery gives the prize in full to each winner.
Australia's official lottery says the man wasn't aware he was a multi-millionaire until he got a call from a lottery official.
Players can register their entries with the lottery, which is how they knew where to find him.
What will he use his new-found riches for?
"I might think about retiring," he said. "First, maybe a new home or a holiday."
Another player also picked the winning numbers, so that individual will also take home $23.3 million.
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