Australians are being urged to check their 20c coins because they could be worth up to $800.

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

With ordinary 20c pieces the number two has a straight base line.

Coin enthusiast Facebook group Detecting Downunder posted a reminder on how to spot the rare coins on Sunday night.

'The way to identify a wavy baseline 20c is it first has to have been minted in 1966,' Detecting Downunder wrote.

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

With ordinary 20c pieces, the number two has a straight base line (pictured above)

'Then look at the bottom section of the number '2' stamped on the side of the coin displaying the platypus.

'On a normal coin, the top and bottom edges of the baseline are straight, while on a wavy baseline coin, the upper edge has a distinctive wave to it.'

Coin expert Matthew Thompson from Town Hall Coins and Collectables in Sydney has previously 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.'

Chris Lawrence from Singleton, NSW, found this 1966 wavy baseline fault coin by 'noodling' - checking through a bag of old coins or a change jar

Coin collector Chris Lawrence from Singleton, 144km northwest of Sydney, said he had found one by 'noodling', the term given to searching through bags of coins.

'It's always good to have one of these bad boys,' Mr Lawrence said on Facebook.

Detecting Downunder advises that if you find one, take it to a reputable coin dealer to have it graded to get a clear idea of its value.

Uncirculated coins are worth more than used coins, while those in very fine condition are worth much more than those in poor condition.

When Daily Mail Australia checked on Monday, most eBay advertisements for the used rare 20c coins were for between $250 and $400 but one seller had listed a certified uncirculated specimen for $4000.

Detecting Downunder said on Facebook that the collectible coins were getting more valuable each year.