The 20 cent coin was first introduced with decimalisation on 14 February 1966. Given that the earlier Australian Pound was equal to 240 pence, and the new Australian Dollar was introduced at two dollars to the pound, the twenty cents denomination was exactly equal to the earlier Australian Florin - both in value and in size (the coins were made of the same dimensions and mass). It is thus a direct descendant of the British Florin.
The original reverse design by Stuart Devlin has not been changed since the introduction. This denomination has since been used infrequently for commemorative designs too.
All of the first issue of 20 cent coins in 1966 was produced at the Royal Mint, London. The Royal Australian Mint did not commence 20 cent coin production until the next financial year (striking coins dated 1966, as listed on its web site). Quantities of 20 cents have also been struck later by the Royal Mint, Llantrisant and the Royal Canadian Mint.
Composition: 75% copper, 25% nickel.