Spear fishing is only legal in Australia on fish which are within the legal size limits, in salt water spear fishing is most popular, as people will often dive or scuba until they see a fish that is clearly over the size limit before making their attack with a spear. In waters without this visibility, or where the size may be questionable, spear fishing should not be used.
If you are interested in spear fishing however, a great way to practice is in suburban rivers, lakes and ponds. Most of these will contain european carp in large numbers. Finding them is fairly simple, they are known to float to the surface and lay idle on warm sunny days, this is the easiest way to spot them in soil stained water.
In lakes which are not very deep, with some visibility it’s often easy to find european carp, however they are extremely shy of humans, so you will need to have some stealth about you. In darker soil stained water without visibility, a trick to find where the fish are is to lightly throw in a handful of rocks into the water, as the rocks fall down next to or on carp at the bottom, the carp will often let out air bubbles which rise to the surface, aiming in this direction is a simple way to spear carp without needing to visually sight them.
Just ensure when spear fishing for carp, that the waters you are spearing in do not have any native fish, as these all have size limits on them. As the fish should be dispatched (killed) after spearing, to avoid breaking any laws ensure the waterways are free from any species with size restrictions, if this is not possible then it is advised to spear only the fish you can see and identify as being of legal size or unrestricted, such as european carp, redfin perch and tench.
In coastal waters, a good alternative for learning to spear fish is on mullet species. These have no size restirctions and are commonly used as bait, in coastal waters there is often much better visibility, so targeting only mullet is a good way to practice, and will also get you some good bait for a traditional rod and reel set up.