The simple answer is 6 months for whole, non-oily fish and 3 months for everything else, which includes oily fish (whole or otherwise), any pieces of fish (such as fillets, cutlets and steaks) and all shellfish, including crustaceans (such as rocklobsters, prawns, crabs) and molluscs (such as mussels, abalone, squid).
The important thing to remember is that these time periods are only valid if the seafood is stored at -18°C or less and some domestic freezers do not get this low, so, if in doubt, check your freezer’s temperature or freeze for shorter periods of time.
Better still, whenever possible, buy fresh Australian seafood and eat it as soon as possible while it’s still at its freshest and tastiest.