1980 5c Australia "Trial Coin"

Hello and welcome.

I have no idea what a "trial coin" is supposed to be in the Australian context, but if it's claiming to be some kind of trial strike (or "lead pull") from an official die, then this is certainly not one of those. A trial strike would not be incuse-mirror-image, it would look exactly like a normal coin. I suppose a "trial strike" from a hub, rather than a die, would be mirror-incuse, but they wouldn't make such an artifact two-sided, it would only be one-sided.

If it's dated 1980, then I would likewise be highly suspicious of any claims as to it being any kind of by-product from Mint testing. The 5 cent piece was not a new coin in 1980; they hadn't changed the design, dies, composition or thickness since 1966. There was therefore nothing that needed "testing" in this fashion.

If you asked me what it is you have, I would assume it's a blob of lead (or some similar metal - perhaps solder) that somebody has pressed two 5 cent pieces into. Certainly not a thing that was made in the Mint.

Did the auction house describe it in any way other than "trial coin"? I am assuming it wasn't a coin auction house, since I can't see Nobles or Roxburys selling this as some kind of Mint trial piece.

It's my experience that many of the non-numismatic auction houses tend to take the vendor's word about auction descriptions of coins - they don't have expert numismatists on-hand, so don't know any better, especially for objects not readily researchable. If the vendor insisted on calling it that, then that's what the auction house will put, no further questions asked.

Lead trials - genuine lead trials, as opposed to yours - are not technically "error coins", since they were deliberately made as part of the Mint's quality control process for ensuring a new coin design's dies look the way they should. The only "error" would be allowing them to escape to the public. But a lead trial is almost always uniface - they only test one die at a time. It's done in a smaller, lower-powered press than the usual coin presses, because the whole point of making a lead pull is that the dies have not yet been fully hardened. This means the dies can be edited or changed if needed, but it also means the dies will deform and become damaged if they're put in a coin press at full strength. It's also why they use lead; it's softer and so puts less stress on the un-hardened dies.

Lead pulls of tokens or medals from private mints are much more common than lead pulls of actual coins from a government mint. Here's an ebay listing of a typical lead pull, of a Bundaberg Numismatic Society medal from the 1960s: https://www.ebay.com/itm/125013038038

Send it back to the auction house for a refund.