You could probably find more info on "19
66 round 50 cent pieces". The 50 cent coins from 1996 were neither round nor silver.

Up until 1966, the currency unit in Australia was the pound, divided like the British pound used to be, into 20 shillings or 240 pence. Australia's circulating predecimal coinage was .500 fine silver. When we "went decimal" in 1966, they replaced silver from the decimal equivalents of all the old coins: the threepence was abolished, the sixpence became 5 cents, the shilling became 10 cents and the florin became 20 cents. All these coins were only struck in cupronickel. A new denomination, 50 cents, was struck from .800 fine silver.
The new coin was unpopular from the beginning; it's size was too close to the 20 cent and its design was too similar to the old florin; the old florin (replaced by the 20 cent coin) had as its design the coat of arms; people naturally assumed the new coin with the coat of arms on it was equivalent to a florin.
Unfortunately, this was also right at the beginning of the silver spike. Not long after they were issued, the new 50 cent coins contained more than 50 cents worth of silver. People hoarded them or shipped them overseas to melt them down rather than spent them, so the government ceased issuing them after 1966. In 1969, a new 50 cent coin, the same design as the old round silver one but struck from cupronickel and dodecagonal (12-sided) in shape. As round 50 cents were returned to the Reserve Bank they were withdrawn from circulation and sent back to the Mint for melting down.
There are still millions of them out there that have survived both the official and unofficial smelters; many old-timers who were around in 1966 have a hoard of the things. They're listed in the Krause catalogue as KM# 67, but around here, unless they're in absolute pristine condition, they're only worth bullion value. Containing .3418 ounces of silver (slightly more than a third of an ounce), the maths for handling small quantities of them is relatively easy: simply divide the number of coins by three and multiply by silver spot price.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis