As someone else mentioned in this thread, carrying a knife is essential for spear fishing, but its illegal to actually carry said knife on shore. A fisheries officer wouldn't necessarily pull you up on it but a police officer might.

spear gun is a mechanical device or object that is capable of imparting propulsive energy to a spear or arrow.
A spear gun may be used to take fish (other than rock lobster) in marine waters (but not waters listed below):
The use or possession of a spear gun is prohibited:
within 30m of any jetty;
within 30m of the mouth of any creek or river (see Definition);
in or on inland waters including Curdies Inlet, Tamboon Inlet, Upper Lake and rivers of Mallacoota Inlet;
in or on the waters of Anderson Inlet, Corner Inlet, Lake Tyers, the Lower Lake at Mallacoota Inlet, Shallow Inlet, Wingan Inlet and the Gippsland Lakes (see below).
The use or possession of a spear gun is prohibited in the waters of the Gippsland Lakes except for those waters that are within:
20m of the 3 rocky groynes off the eastern end of Rigby Island;
20m of the 'barrier' in Hopetoun Channel located to the west of the barrier Landing;
50m of each of the compass beacons in Eagle Point Bay;
imaginary lines running from Wattle Point to Trouser Point and from Waddy Point to Pelican Point in Lake Victoria (see map below).