The round fifty cent coin was the highest-denomination and largest diameter coin of Australian decimal coins when issued. It was minted in 1966 and due to the rising cost of silver, it was never minted again. It was replaced by a dodecagonal 50 cent coin in 1969, which retained the Australian Coat of Arms.
The original round coins were made of 80% silver and 20% copper. As the value of silver grew higher, the coins’ bullion value became worth more than their face value. This resulted in the government of the day withdrawing them from circulation. 36.45 million coins were initially minted; however, only 14 million were ever put into circulation.
Out of the total 14 million coins that were put into circulation, only a fraction ever made their way back to the mint and remain “missing” to this date. Today, these very same coins are selling for $15 – $20 each on eBay.
Shortly after being minted, the rising price of silver made these coins extremely sought after by criminals. Most were illegally smuggled out of the country and found their way to Hong Kong, where they were melted down for their silver content.
Even back in 1968, as seen in this video by the ABC, the problem was already well under way and out of control.