A shop in Kalgoorlie shared pictures of the rock online, estimated to be worth A$100,000 (£54,000; $69,000).
The unidentified man was an experienced local hobbyist, shop owner Matt Cook, told the BBC. Finds of this scale by prospectors are known to happen a few times a year, experts say.
About three-quarters of the gold mined in Australia is produced in and around the Kalgoorlie region.
Mr Cook, who owns a shop selling supplies to gold prospectors, said the man detected the piece on some saltbush flats, about 45cm (18 inches) below the surface.
"He walked into my shop and showed me the nugget in his hand with a big smile on my face," Mr Cook told the BBC.
"It just a bit bigger than a packet of smokes, and the density of it was incredible, so heavy."
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