The Australian 20 cent coin was first introduced with decimalisation on 14 February 1966 when it replaced the pre-decimal Australian florin, to which it is equal in size and mass. The original reverse design by Stuart Devlin has not been changed since its introduction.
The denomination is occasionally used by both the Royal Australian Mint and the Perth Mint to issue collector coins, which are legal tender in Australia - such as this one.
This coin was issued as part of the 1999 Masterpieces in Silver - Coins of the 20th Century: Memories six-coin set, highlighting important times in Australian history (mintage limit 15,000, issue price A$125.00). The reverses of each coin show a historic Australian coin design, in mirror-finish proof; the denomination and value are on the obverse.
The five cents coin depicts a 1939 kangaroo-type halfpenny.
The ten cents coin depicts a 1930 penny.
The twenty cents coin depicts a 1942 threepence with no mint mark.
A shortage of coins saw Australia turn towards the San Francisco and Denver mints to produce 24 million 1942 threepences. While these had S or D mint marks to identify their origin, the real rarities are 1942 threepences with no mint mark – a group of only 528,000 struck by the Melbourne Mint in Australia.
The fifty cents coin depicts a 1918 sixpence.
The one dollar shows a 1940 shilling.
The two dollar coin shows a 1920 sovereign.
These coins are Non-Circulating Legal Tender (collector issue); composition: fine silver (99.9%). No coins of this type have been released into circulation. | 
Search for Australia: Twenty Cents 1999 (Silver, Threepence) on eBay 