The man, who doesn't want to be named, made the discovery in Victoria's goldfields - which were the heart of Australia's gold rush in the 1800s.
Darren Kamp, who valued and bought the specimen, said it was the biggest he'd seen in his 43-year career.
"I was just gobsmacked... It's a once in a lifetime find," he told the BBC.
Kamp hadn't thought too much of it when a man wearing a large backpack walked into his prospecting store in Geelong, about an hour south-west of Melbourne.
Normally people came in with fools gold or other rocks that looked like gold, Kamp said.
"But he pulled this rock out and as he dropped it into my hand, he said, 'Do you think there's A$10,000 worth in it?'"
"I looked at him and said, 'Try A$100,000'."
The man then proceeded to tell him the rock in Kamp's hand was only half the find.
All up, the 4.6kg rock contained about 2.6kg of gold.
After having it valued, Kamp bought it off him.
He said the lucky man was looking forward to spending the windfall on his family: "He said to me, 'Oh the wife will be happy'."
While discoveries like this are rare, Australia is estimated to have the world's largest gold reserves and many of the world's biggest nuggets were found in Australia.