You have certain rights under the CRIMES ACT 1961 to defend your property from offences such as theft, burglary and trespass (see below). You will therefore not be criminally liable for doing so.
The right to defend your property is similar to that of self-defence against a physical attack, in that you are justified in using "reasonable force" (see How to legally defend yourself against an attacker).
You, and anyone who assists you, are justified in using reasonable force in order to prevent someone taking an item of your property, or in order to retake it. You are justified even if the person attempting to take the item thought he or she had the right to do so.
If an item is not yours, and you are not acting under the authority of its owner, then you are not protected from criminal liability in using reasonable force to defend it.
You and anyone acting under your authority are justified in using such force as is necessary to prevent a person breaking and entering into your home. This includes taking action to prevent a burglary continuing if you discover that burglars are already in your house.
The defence is available to you only if you believed, on reasonable and probable grounds, that there was no legal justification for the breaking and entering.
You are justified in using reasonable force to prevent anyone from trespassing on any land or buildings of which you are in possession. This applies also to anyone assisting you or acting under your authority.
In the situations above, reasonable force does not include striking the other person, nor inflicting bodily harm.
It is difficult to define what is reasonable force, as each situation will be different. But essentially you must use force that is proportionate to the harm that you are trying to prevent.
As the right to use reasonable force extends to anyone acting under your authority, this would include one of your employees trying to prevent someone taking your property.
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