The Australian gold half gram coin is a mini-coin format occasionally used to complement issues in larger sizes. Uniquely, in Australia there are two mints authorised to strike legal tender: the Royal Australian Mint (which also makes the country's circulating coinage) and the Perth Mint which only makes collector and bullion coins, as well as other bullion products.
Both mints endeavour to create coins with attractive designs, and have long-running bullion series of coins with different themes. As part of these, they sometimes issue a mini-version of the theme's annual design.
This coin is part of the long-running Australian Kangaroo bullion coin series by the Perth Mint, and is a "numismatic" issue with a one-year design. Due to its small size, its reverse represents only part of the design of the larger coins.
The Mini Roo was introduced in 2010 with the intention of allowing more people to have the opportunity for gold ownership by providing a very low price point.
The mint says about it:
The Australian kangaroo just got cuter with the release of the button-sized 2013 Mini Roo coin. Extremely affordable, this release makes the perfect addition to any fauna themed coin collection. | 
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