Different 'silver' coins contain differing amounts.
Kookaburras are 99.9% silver
Florins are 92.5% (=sterling)
Shillings, sixpences and threepences (1953-1963) are all 50% silver.
The 1966 50 cent coin is 80% silver. It's the only Australian decimal coin that contains any silver - obviously apart from the bullion and proof silvercoins minted by the Perth Mint and the ARM as Silver Coins.