The case of an Adelaide mother who was turned away from a convenience store for trying to pay with $10 worth of 50-cent coins highlighted just one of the laws around Australia's coinage you probably didn't know about.

Amy Bivone took to Facebook to vent her frustration after she was turned away from the On The Run (OTR) shop after trying to buy milk and bread for her children.

"$10 worth of 50c pieces to get 2x 2ltr Milks and 2x Breads for my kids at OTR so I didn't have to lug my 2yr old and 11 month old through the shops and have the car out in the heat just for a simple OTR job," she wrote.

Amy Bivone said she was turned away from the service station when she pulled out a stash of coins to pay for her groceries. ( Facebook: Amy Bivone )

The servo responded by citing the Australian Currency Act 1965 (section 16) that states coins are not legal tender if they exceed:

20 cents where 1-cent and/or 2-cent coins are offered (these coins have been withdrawn from circulation, but are still legal tender)

20 cents where 1-cent and/or 2-cent coins are offered (these coins have been withdrawn from circulation, but are still legal tender) $5 if any combination of 5-cent, 10-cent, 20-cent and/or 50-cent coins are offered

$5 if any combination of 5-cent, 10-cent, 20-cent and/or 50-cent coins are offered 10 times the face value of the coin if $1 or $2 coins are offered

What does that mean?

It means if someone wants to pay with 5-cent coins, they can be told they can only pay up to $5 worth — with any more than that not deemed legal tender.

However, that is at the discretion of the person being paid. If they are happy to accept payment of $100 in 5-cent pieces, there is no problem.

And yes, 1-cent and 2-cent pieces are still Australian legal tender, but they are not considered as "currency" (or, money that is officially released for circulation).

This means that you can take your old 1-cent and 2-cent coins to the bank and exchange them for currency totalling the same face value.

In fact, only two Australian coins — the holey dollar and dump coins — have had their legal tender status removed.

These were Spanish dollars converted for use in Australia from 1813, when New South Wales governor Lachlan Macquarie imported 40,000 of them and ordered a convicted coin forger to punch a circle from the middle of each coin.

The dump was the piece removed from the centre while the remaining ring was known as the holey dollar.

They were outlawed by the British government in 1825, but all other coins remain legal tender.

Can you use pennies and shillings to buy a coffee?

Yes. In theory. Only the holey dollar and dump coins have been demonetised.

But with the value given to out-of-circulation coins, you may be handing over shrapnel that is far more valuable to collectors.

A penny could be used today but is probably worth more to a collector than a shop-keeper. ( Supplied: Royal Australian Mint )

"Some coins which are no longer in circulation, such as Australia's pre-decimal coins and the 1 and 2-cent coins, are still legal tender," the Royal Australian Mint explained.

The current monetary values of old coins which were removed when decimal currency came in on February 14, 1966, are set in the Currency Act 1965.

One sovereign or pound is worth $2, a shilling 10 cents, and a single penny is now valued at five-sixths of a cent.

The 1930 proof penny is Australia's rarest coin, with only six still in existence. Three are in private collections, and the others are kept in the Museum of Victoria, the National Gallery of South Australia and the British Museum.

Proof coins are collector pieces that were never intended for everyday use.

If you had used one of these pennies at a shop today it would have counted for five-sixths of a cent, a long way short of the $225,000 for which one was sold in 1998.

Many shillings minted between 1910 and 1920 are worth at least $8, and those in high demand and good condition could be worth more than $7,500.

There is another problem. Many people wouldn't recognise pre-decimal currency and could be hesitant about accepting it. And they may use the limits in section 16 of the Currency Act if you try to use 50 shillings to settle a $5 bill.

So the possibility exists but there's no law compelling anyone to take old coins.

How many old coins are still around?

About 2.4 billion coins were minted between 1910, when a national Australian currency was formed, and 1964 — two years before the switch to the coins we recognise today.

The Reserve Bank of Australia keeps records about the circulation of banknotes, however it is unclear how many of those early coins are still around, as it is likely some were melted down to be reused.