In criminal law, on any country and that includes Australia, self defence is mainly evaluated according to what happened when it was used.

It's summed up in this phrase:

"the defendant honestly believed the threat to be imminent and made an objectively reasonable and proportionate response to the circumstances"

So imagine someone attacked you and you used mace to defend yourself and in doing so you go to such extremes that you kill the attacker. That might be considered excessive self defense.

One thing is to consider the posession of such devices (mace, tasers, etc) as illegal, another if the use of them might be considered for grounds of excessive self defence.

In my research I found this reggarding the posession of such devices:

In Western Australia, it is legal for a person to carry pepper spray for lawful defense, if that person has, on reasonable grounds, a suspicion or belief that he or she will require the pepper spray to defend himself or herself. However, the person found carrying the pepper spray carries the burden of proving a "reasonable belief or suspicion" rather than the prosecution. In all other states and territories in Australia, pepper spray is considered illegal"

Self-defense is absolutely legal as long as it is justifiable and the aggressor receives little or no punishment. Here's a complete reference to self-defense:
http://www.blackbeltforlife.com/docs/Selfdefense_meaning.pdf

well, i guess that self defence is always a big issues because if you hurt someone in respect of the self defence then govt. arrest you for charge. In australia they have made some laws against it and rise and shine with the self defence vision to protect them selves and others if needed.

http://myselfdefensestore.com/