Robert Hacon discovered the 1.6 metre fossilised jaw of a Kronosaurus Queenslandicus, an 11 metre long apex predator, while working on his property.
The Queenslandicus had a crocodile-like head, a body with four powerful flippers, jaws twice as powerful as those of a saltwater crocodile and curved teeth the size of bananas.
"I was out poisoning prickly Acacia and saw some objects shining in the distance," Hacon was quoted as saying by the Brisbane Times.
Hacon then contacted Dr Timothy Holland, curator of Richmond's Kronosaurus Korner Museum.
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"The specimen represents the most complete mandible of a Kronosaurus Queenslandicus in the world, with most other examples being weathered, crushed or incomplete. This is the real deal," Holland noted.
The first recorded Kronosaurus Queenslandicus remains were discovered near Hughenden in 1899.
However, a near-complete skeleton at Harvard University's Museum of Comparative Zoology is the most famous.
"The mandible is just so robust, measuring 18cm deep in some places. The scary thing is that this creature wasn't even an adult when it died - it still had a lot of growing to do," Holland added.