Australia is relatively progressive with currency, having made several sensible concessions to practicality.
Copper coins
Until the 1980s, Australia had small copper one and two cent pieces. However, inflation gradually made such amounts of money of negligible value. So, these coins were eliminated, with the smallest coin being the silver-coloured five cent piece. Cash transactions are rounded to the nearest five cents.
This was a good move, because these coins were annoying, and worthless. Due to the rounding up or down to the nearest transaction, after a reasonable number of transactions, the few cents saved on some transactions will negate losses on others, so no-one gets ripped off, no pesky coins, everyone is happy. Of course the many transactions done electronically by EFTPOS or credit card these days don't have to be rounded.
Gold coins
Australia used to have notes for $1, $2, $5, $10, $20, $50 and $100. In the early 80s, the $1 note was replaced with a coin, to reflect the the fact that a dollar wasn't such a substantial sum of money any more. In 1988, the $2 note met the same fate. Both were replaced with "gold coins" with a distinctive colour to differentiate them from the silver "shrapnel" coins of little value (5c, 10c, 20c, 50c).
Plastic Notes
During the early 1990s, the remaining paper notes were phased out in favor of new polymer notes, one denomination at a time. These notes are far more durable than the paper ones, and extremely difficult to forge. The paper notes were becoming easier to forge as colour photocopiers and colour laser printers became more common.
The plastic notes have many security features, such as a 7 pointed star with 3 points printed on one side and 4 on the other. When held up to the light, you they should line up perfectly to form the star. A forgery is unlikely to have the sides line up that well. The phrase "TEN DOLLARS" or whatever is printed in microscopic type, barely visible to the naked eye, that is likely to be blurred in a fake under a microscope. There is also a transparent window to verify that the plastic is clear like a real note, as well as raised ink providing a distinctive texture.
The plastic from which the notes are made is very tough, and was a newly developed polymer compound. You can grab a note at either end and pull as hard as you can without any effect. You have to cut it with a blade, then it will tear, a little like fabric. As a result, the most circulated notes last for up to 5 or so years, rather than around 6 - 8 weeks when they were paper. This plastic is purpose made for notes, and unlike any other plastics in common use. It is a little like overhead projector transparency or film negative plastic, but thinner, more flexible, and tougher.
The notes are all the same height, but get longer as they increase in value. They feature complex and colouful designs of national icons. Each note has a distinct colour to avoid confusion - $5 (purple), $10 (blue), $20 (red), $50 (green), $100 (white). These colours are the same as were used for the paper notes, and are kept consistent whenever new designs come out.
As a result of these changes, Australia has some of the most practical currency in the world. Making worthless coins obsolete, worthless notes coins, the basic scale (notes of considerable value, coins just for making up change) has been preserved. The distinct notes are nigh impossible to confuse or forge. The coins are colour coded.
Australia is recognised as a leader in this area, and actually manufactures the currency for a number of pacific nations.