The Australian one ounce silver piece (abbreviated as 1 oz and designated with Ag for "silver") is a bullion and commemorative 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 coin is part of the Australian Brumby series of bullion coins, which continues the Australian Stock Horse range.
The mint says about it:
Brumby is the term for Australia’s wild horse. Brumbies that most famously roam the Australian Alps are descended from lost, abandoned or escaped horses that in many cases belonged to early colonial settlers like James Brumby. Free-roaming, independent and naturally adapted to its rugged environment, the brumby has become a national icon seen by many as symbolic of our national character. | 
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