The Australian half ounce silver piece (abbreviated as 1/2 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 Perth Mint's Australian Kangaroo series and has the same design as the flagship silver bullion ounce. However, it is a "numismatic" (special) proof issue for the ANDA Brisbane Money Expo in 2018.
The Mint says about it:
The Perth Mint is proud to present this exclusive 1/2oz silver release from the Australian Kangaroo Coin Series, the Brisbane Money Expo ANDA Special Australian Kangaroo 2018 1/2oz Silver Proof Coin. The coin is struck by The Perth Mint from 1/2oz of 99.99% pure silver in proof quality. Its reverse portrays The Perth Mint’s classic depiction of an Australian icon, the bounding red kangaroo surrounded by stylised rays of sunlight. | 
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