• The coin is known as a 'mule', as it is the product of two different species of parents. A rare 20c piece made in 1966 with a wavy baseline fault on the number '2' is now selling for up to $2000.
• Dedicated to rare Australian coins. First in the United States.

Barron's Penta consults with an expert at Stack's Bowers, America's oldest rare-coin auctioneer.

One other type of rare Australian coin to look out for in your change are errors. Most commonly you might find a coin with a cud. For some reason $1 and $2 coins seem to occur most frequently with cuds. Less often you might be lucky enough to find a mis-struck coin such as an out-of-collar 5c or 10c or a coin with a minor clip.

A coin expert has urged people to pay more attention to their spare change before giving it away, revealing some rare 5c, 20c and $1 coins could fetch you a whopping $20,000.Source:Supplied

A coin expert has urged people to pay more attention to their spare change before giving it away, revealing some rare 5c, 20c and $1 coins could fetch you a whopping $20,000.

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A $1 coin recently was put up for auction at $4000, with similar coins – defined by small minting mistakes and known as a ‘mule’ – among the most sought after by collectors around the world.

“We don’t know how many are (in circulation), but they originally started showing up in Perth,” coin expert David Jobson told The Morning Show today.

“Somebody’s missed it at The Mint, and people have gotten lucky.”

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A rare $1 coin that's been put up for auction at a whopping $4000. Picture: The Morning ShowSource:Supplied

This five-cent coin, where both sides are heads, is worth at least $1000. Picture: The Morning ShowSource:Supplied

Mr Jobson said the value of the coin depends on its quality – a really high quality coin could go for $4000 or $5000, while one that’s poor quality is only worth about $300 or $400.

Among his personal collection is a five-cent coin where both sides are heads – what he called a “striking error”.

“In 2007, they accidentally produced a five-cent piece that had the Queen’s head on both sides. So it’s quite a striking error, and they’re a lot easier to notice in your change,” he explained.

“Value-wise, it starts off at about $1000, depending on the quality – and it goes up to about $4000 for really nice ones.”

He also recently sold a 20-cent coin with scalloped edges – caused by a printing mistake with a Hong Kong coin – valued at a staggering $20,000.

“In 1981, The Royal Mint in London produced our 20-cent pieces,” Mr Jobson said.

“They also produced the Hong Kong $2 coins. So somebody accidentally mixed the $2 scalloped-edge coin with our 20-cent dies, and they produced that coin.

“It’s very valuable. There are less than 10 that are known to exist, and they value anywhere between $15,000 and $20,000.”

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Only ten of these 20-cent coins are believed to be in circulation, and are worth between $15,000 and $20,000. Picture: The Morning ShowSource:Supplied

Mr Jobson advised people hang onto any strange-looking loose change – which can be worth a little bit of money if you have multiple versions of the same coin.

Rare Australian Coins For Sale

“They’re definitely worth hanging onto. If you find something rare, you need to speak to an expert and show them,” he said.

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“Take a really good photo, send it off to your local coin dealer and they should be able to tell you what it is.”