This defence is used when the Accused, at the time of committing the offence/s, was suffering from an mental impairment such that:
- They did not understand what they were doing; and
- They did not know what they were doing was wrong.
Under s. 20 of the Crimes (Mental Impairment and Unfitness to be Tried) Act 1997:
A person is presumed to be not suffering from a mental impairment unless it is proved by the defence.
The standard upon which the defence must prove mental impairment is ‘on the balance of probabilities’.