Although most forms of physical self-defence products are illegal in Australia, there are two options that are legal and can quite literally save your life. A bright torch, and a device that is capable of making an incredibly loud, preferably high pitched sound.
While these may not sound like they offer much physical protection, they can provide vital time to escape. A bright torch when shined in an attacker's eyes can temporarily blind them, while a device which is capable of making a significant amount of noise (such as the above mentioned personal protective alarm) can disorient your attacker.
While you'll probably only buy yourself a short amount of time, it's better than nothing. Plus, as you're not doing damage to your assailant, you will, in most cases, be protected by self-defence laws. These items are considered legal to carry, as they're everyday products, and you will not be penalised for having them on your person.
We do also recommend carrying a lanyard with you. This is because swinging it around can act as a deterrent to would be attackers. Plus, you'll have a longer range when fighting back. However it is important to remember that anything that you carry can be used against you if it is taken from you.