$2

Collector-bill … the $2 note with asterisk is valued at $6150.

One- and two-dollar notes, long replaced by gold coins, are still worth something today. Most can be exchanged for their face value but a few, very few, are worth a lot more than that.

Ten thousand dollars in some cases.

A kind of cryptic code applies here. Prefix letters - clearly seen in the bottom left-hand corners - indicate when the notes were issued to the public. Those from the first and last issues are more valuable than others and are now seen by collectors as a good investment.

$1

The $1 notes are a consecutive AAA-prefixed pair valued at $2975.

The serial numbers after the prefix indicate its order of printing. Those numbered below 1000 are also considered more valuable.

The most desirable of all are consecutively numbered pairs (or more if you're lucky). For example, a consecutive pair of 1966 $1 notes bearing the first AAA prefix is now valued at $2975. Sydney currency dealer John Pettit Rare Banknotes features a pair of these in uncirculated (mint) condition in his latest catalogue.

A single $1 note, also AAA but with a serial number lower than 1000, is also listed, at $1650.

Pettit also offers a pair of 1966 $2 notes - first prefix and consecutively numbered - with a value of $6750. A single $2 note (first prefix, numbered under 1000) is worth $3000.

Also in demand are star notes.

These are marked with a star, or asterisk to be correct, after the serial number. This indicates that the note was issued to replace one damaged in the production process. Star notes can be as rare as one in 1921 in certain prefix listings and are valued accordingly.

A combination of all these rarity factors adds greatly to its value. Pettit lists a 1966 $2 note, with the first prefix ZFA and the prized star, for $6150. Even mid-prefix star notes are worth more than $5000 in uncirculated condition.

These values are provided by the industry bible, Renniks Australian Coin and Banknote Values. In its latest editions, this book recognised the new collecting trend by including separate listings for all prefix varieties.

Larger-denomination notes are also collected, although a consecutive pair of first-prefix $5 notes is worth about the same as an equivalent pair of $1 notes.

A final factor adds to value: the signatures on the notes. Coombs and Randall (that's ''Nugget'' Coombs, a former governor of the Reserve Bank and Whitlam adviser) is considered the rarest combination because they only served together briefly, in 1968.

The highest-value $1 note listed by Renniks is a low-numbered ZAH prefix issued by Coombs and Randall. These are worth $15,000 each.

The highest-value $2 note is again by Coombs and Randall. Those with prefixes ZFH and ZFK are worth $12,000. In theory there should be plenty of these notes around.

Many were kept as souvenirs when decimal currency was introduced in 1966. When the $1 note was withdrawn in 1984, plenty of the last issues were also kept. Finding them in uncirculated condition is the tricky bit.

Paper notes were regarded as expendable, so it seems amazing that any have survived in untouched condition after 50 years or more. These are sometimes found hidden in the pages of old books or even in unopened pay packets that have slipped down the backs of chairs.

The collecting of Australia's decimal banknotes is a new phenomenon, yet values have increased dramatically over the past 10 years.

The expectation is that prices will continue to rise steadily over the next few decades as more collectors join in the fad.

Wise investors store their treasures in envelopes made of Mylar, a clear-plastic material used by museums to preserve important paper documents. Otherwise your investment could turn into powder by the time you decide to sell.