A secret detail on a small number of 20c coins minted in one year could be worth a small fortune - and one could be in your wallet.

Tik Tok channel The History of Money said about 0.5 per cent of 20c pieces made in 2004 could be worth $8 each to collectors.

The Perth video blogger, known as Joelo, explained the valuable coins have a bigger image of the Queen's head on the back.

Pictured: The coins look identical from the front Pictured: One coin has a larger head on the back

Pictured: The coins look identical on the front, but one has a larger head on the back

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Coins that have a larger head were also imprinted closer to the edge and have a pointy letter A in the text around the rim.

'At first glance, these 2004 20c coins are identical,' the coin expert said.

'But the one on the left has a larger head than the one on the right.' 

Out of about 72 million 20c coins produced that year, only about 400,000 have larger heads.

Each piece with the unique feature could be worth up to $8 if kept in good condition. 

'Handy addition to your collection,' he said.

A coin with a larger head (pictured right) are worth about $8, as only half a per cent of 20c coins minted have the anomaly

A coin with a larger head (pictured right) are worth about $8, as only half a per cent of 20c coins minted have the anomaly

Pictured: The A is pointy on rare coins Pictured: The text is closer to the rim

Pictured: The 'A' is pointy on the large head and flat on the small head and is closer to the rim

HOW TO TELL IF YOU HAVE A RARE 20c COIN

A rare coin will have a larger head than ordinary coins.

The 'A' is pointy on the large head and flat on the small head.

The large head is also closer to the edge.

Source: Tik Tok

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Excited viewers shared their thoughts in the comments to the video.

'Eight dollars for a 20c coin,' one user wrote with two surprised emojis.   

Australians should also look out for 20-cent coins from 1966 because the number 2 has a wavy baseline.

Ordinary 20c pieces have a straight baseline on the number two. 

The anomaly could be worth up to $800.

Coin expert Matthew Thompson from Town Hall Coins and Collectables in Sydney told Daily Mail Australia that most people pass on rare coins by accident - so they should be looking for anything out of place.

'People don't expect institutions like the Mint to make mistakes,' he said.

'But from time to time things can go awry. If you see mistakes on a coin, if you have something interesting, odd or out of place, then other people are likely to find it interesting, too - that's why people collect.' 

The rare coins, minted in 1966, are highly sought after by collectors because the number 2 has a wavy baseline

The rare coins, minted in 1966, are highly sought after by collectors because the number 2 has a wavy baseline 

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