2000 Millennium incuse flag 50 cent
In 2000, the Royal Australian Mint issues a commemorative 50c piece for the millennium. The reverse design showed an Australian flag with the phrase MILLENNIUM YEAR and 50 CENTS. 16,630,000 of the coins were issued for circulation. At some point in 2006 it was noticed by Drew Jackson that there were two varieties of the reverse design. One type has a raised St George's cross and the other type has a recessed St George's cross (p14, McConelly, The Coin Seriously Lacking? in Australasian Coin and Banknote Magazine, June 2006).
It was soon apparent that the recessed flag coins, more commonly known as the 'incused flag' were much less common. The Royal Australian Mint advised that approximately 200,000 of this variety were struck (p143, McDonald, Pocket Guide to Australian Coins and Banknotes, 2017). It was probably the output of a single die and the approximate mintage was probably based on the average output of a single 50c reverse die.
The true rarity will probably never be known but an analysis of examples in circulation shows that the given approximate figure of 200,000 coins is reasonable: in a sample of 212 2000 millennium 50c pieces, 0.94% of the coins had the incused flag. The would suggest around 157,000 incused flag 50c pieces were struck and released into circulation, making it by far Australia's rarest circulation 50c piece. No incused flag coins are known in mint sets so most examples are at best lightly circulated.