This is an Adelaide Assay Office (Australia ) Gold Ingot - this one is at the Royal Australian Mint and worth in the region of 600,000 to 700,000 Australian dollars. I think there are only 6 or 7 known to have survived.
They were produced in Adelaide in the early 1850's

Fascinating - I wonder why this size and not a coin or a larger bar? Could they have been a made to order thing where they just smelted and formed a bar of a depositors gold?

Nice picture - I didn't bother to get a closeup of that ingot.

Here is a picture with the description to whose you are interested in reading it's history

http://www.gxseries.com/gallery/index.php?...07%2Fmint19.jpg

Also taken in the Royal Australian Mint.