This is a specimen of the Triassic fish Cleithrolepis, and was found near Sydney in NSW.
These fish are reasonably common in this area, and this is a fine specimen from Hornsby Heights.
You can see the eye and the fine features of the fins. These lived in rivers, billabongs and lakes in amongst the large braided river system that deposited the Hawkesbury Sandstone.
There are found now in what we call shale lenses, which are the remains of these lakes. Fine sediments like clays are deposited; after many millions of years they become shales.