Australia's most valuable coins

Check your pockets - this 20-cent piece has a tiny alteration that makes it worth thousands of dollars.

When Australia switched to decimal currency in 1966, the country sought the aid of Royal Mint in London to create the vast stocks of new coinage needed.

According to collectables website Downies, a "tiny number" of the 30 million 20-cent pieces it struck in 1966 were distinguished by an upward curve on the baseline of the numeral "2".

This alteration makes it "one of Australia's rarest decimal coin types", especially if it's in good condition. Downies values a top-line version of the Wavy "2" 20c at a sale value of $4750.

Of the 30 million 1966 20c coins struck at the Mint, a tiny number were distinguished by the obvious upward curve on the top of the baseline in the "2".

With most 1966 Wavy "2" 20c coins suffering extensive circulation, this famous Australian variety is very rare in top grade.

We now know how the new monarch will look on Australian coins after the Mint released the finalised effigy of King Charles III.

He will gradually replace his mother, Queen Elizabeth II, on all Australian coinage from towards the end of this year.

Will the first King Charles dollars grow in value over the years?

It's hard to say - after all, as the following Aussie coins show, sometimes an apparently normal handful of change can be worth thousands of dollars.

Website Numista.com estimates there are just between 6000 to 7000 copies of this 2000 $1 coin in circulation. And if you've got one, coin expert Joel Kandiah says it could be worth up to $3000.

That's all because this batch of $1 coins was struck on the obverse (ie, the side featuring the monarch's effigy) not with the dollar coin die but the 10-cent coin die. As seen in this image courtesy of Uni, it creates a double rim around the coin's edge, making it easy to spot.

This 1930 penny in "very fine" condition is for sale at Downies for a whopping $42,500.

Only about 1500 examples of this Depression-era coin are thought to still exist, and they typically fetch high prices wherever they go to auction.

For $24,950, this 1852 gold Adelaide pound could be yours.

Predating Federation by decades, it's one of Australia's rarest coins, with just 200 examples known, Downies said.

It was minted as an emergency currency following Australia's 19th-century gold rush.

This looks a lot like a regular 50-cent piece, yet it's worth $14,750. Why? Because it's from 1988.

In the 200th year since the arrival of the First Fleet, it was decided that 50-cent coins would commemorate the anniversary, and so the standard coat-of-arms 50-cent design was shelved for a year.

Well, except for this one at least, which managed to slip past the keeper with a 1988 date on its back.

Fewer than a million gold sovereigns were struck for 1921 in the reign of King George V.

Uncirculated versions are for sale at $4500.

Sometimes, even when a coin is worth nothing it can become worth a lot - $3950 in this case.

With two Queen Elizabeths, this mis-struck token has a reversed portrait on one side.

Another coin that predates Australia's federation is this 1899 golden sovereign featuring the effigy of Queen Victoria at the height of the British Empire. It's worth $5500.

This could have been our $1 coin.

The "goose dollar", designed by Andor Mészáros, won a competition by Australian Coin Review for potential dollar coin designs in 1967. The Review submitted the design to the Commonwealth, but it was rejected. The Review had a number privately minted regardless.

In this print quality, it's worth $4,499.99.

A very rare well-preserved Australia's first ever florin coin from 1910, this is worth $4500.

A florin is two shillings, or 24 pence, which equals one tenth of a pound.

Is it any wonder we switched to decimal?

Just over half a million 1908 Australian half-sovereigns were ever struck, almost guaranteeing them a future as collector's items. Featuring the effigy of Edward VII - the great-great-grandfather of Charles III - one this well preserved is worth $1995.