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 one-ounce silver bullion coin is part of the Perth Mint's Australian Brumby coin range, which features a new reverse design every year.
The Mint says about it:
According to tradition, when soldier and pastoralist James Brumby was transferred from Sydney to Van Diemen’s Land in 1801, he was forced to release a number of horses into the bush. Known as "brumbies", their descendants continue to roam the Australian Alps today. Hardy and independent, they are seen by many as symbolic of the Australian national character. | 
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