The Australian one-ounce gold piece (abbreviated as 1 oz and designated with Au for "gold") is a bullion coin format. 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 to introduce new designs and themes often, in order to raise the numismatic value of the coins over the value of previous metal used.
This one-ounce gold coin with a face value of $100 is the third gold issue in the Perth Mint's Australian Brumby (wild horse) coin range, which features a new reverse design every year; the gold ounce repeats the design of the previous year's silver ounce.
The mint says about it:
Brumbies are descended from stock thought to have escaped from their early colonial owners. Energetic, independent, and ideally adapted to the rugged conditions of the Australian Alps, brumbies are seen by many as symbolic of our national character. | 
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