Australians could be hanging on to a $1 that could be worth thousands of dollars, and not even know it!

A mother in Melbourne has posted on the Facebook group Melbourne with Kidz that she found a “mule” dollar coin from the year 2000.

These coins were produced due to a technical error by the Australian Mint in Canberra twenty years ago.

Mule dollars are slightly thicker than a regular $1 coin in appearance and also have a double rum around the Queen’s head.

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“It’s a small number of the year 2000 $1 coins that had been minted using the incorrect obverse die (heads side) and released into circulation by mistake and only discovered a year or two later,” the mum wrote after doing some research.

“With just a 1.4 millimetre difference in diameter between the 10 cent and $1 coin, you can clearly see a double rim circle going around the edges of the coin.”

The Daily Mail reported that there are just 6000 coins that were minted incorrectly.

A few of these coins have been placed on eBay at a value ranging from $700 to $5000.

“Check your change and empty out the kids piggy bank! You could be sitting on a winner,” the mum said.

This article originally appeared on Over60.