The new fossil comes from the same locality that produced the skeleton of Isisfordia duncani, the world's first modern crocodilian, and is estimated to be between 98 and 95 million years old.
The Isisford tarpon was found in the mid-1990s by then local grazier Ian Duncan.
The beautifully preserved, articulated skeleton was entombed in a large sandstone boulder, close to where Mr Duncan found the first pieces of Isisfordia.
“I'd been poking around in the creek bed looking for more pieces of the croc when I came across a large bolder,” Mr Duncan said.
“There weren't any bones sticking out, but it had the right kind of look about it, so I thought I'd try and wedge it open with my crowbar.”
The boulder split along the plane of weakness created by the skeleton, and opened to reveal the beautifully preserved fossil inside.
The preserved portion of the fish includes the back of the head and front half of the body.
The complete fish is estimated to have been over a metre in length.
It would have looked something like this (modern day tarpon Megalops cyprinoides):