They were silver Spanish coins adapted for the new colony.

As Sydney Cove expanded from a ramshackle collection of buildings into a town, Governor Lachlan Macquarie decided the new colony needed its own currency.

He decided to import 40,000 Spanish Silver Dollars and have the centre punched out.

This middle bit and the outer bit were both turned into separate coins.

A convict, William Henshall, did the job. He was an expert, having been transported for forgery and counterfeiting.

The central plug of the imported Spanish coin - the Dump as it came to be known in colonial NSW - had a crown stamped onto one side and the value on the other - 15 pence (or one shilling and three pence as it would have been then).

The outer coin with the hole came to be known as the Holey Dollar. It was stamped "New South Wales" on one side and "Five Shillings" on the other.

The Holey Dollar and Dump remained as Australian currency until 1829 when Britain decreed that New South Wales would start using British money - pounds, shillings and pence.

Before the Holey Dollars and Dumps, anything went. In the earliest years of the colony, after the arrival of the First Fleet in 1788, British, Dutch, Indian and Portuguese coins were used for transactions.

And rum. The spirit was used to buy goods until it was banned in 1806 by Governor Bligh (he of the Mutiny on the Bounty).

The Holy Dollars and Dumps turned out to be only temporary. As the colony and economy grew even more, Britain decided that British currency should be used - pounds, shillings and pence.

Since then, Holey Dollars and Dumps have been turning up every now and again though they remain rare. Experts estimate that only 350 Holey Dollars and 1500 Dumps remain.

"They do look rather unusual - like a washer," said the Royal Australian Mint's collections manager, Holly Anderson.

"They have been found in some very unusual places. One was found in an old Hills Hoist clothes line. The problem with that one is that every time the clothes line went around it shaved about a thousand dollars of value off it."

Her favourite is the oldest one they have on display. It's a Spanish coin dated 1757, when the Spanish king was Ferdinand VII.

It's hard to put a current value on the collection. Who knows what would be bid at auction for the collection, but one estimate puts it at $7 million.

Holly Anderson says only that it would help your property ambitions if you found one.

"Finding a Holey Dollar, depending on its condition, on the effigy of the Spanish monarch and any of the history around it can be very, very worthwhile helping you buy a house."