In a letter sent on Tuesday morning to the NSW attorney general, Mark Speakman, and other MPs, Rape and Domestic Violence Services Australia urged amendments to the laws, which are to be debated in parliament this week.
The service, which runs the NSW Rape Crisis Centre, says it consulted with other providers within the sector, as well as academics and psychologists, before preparing the letter.
The definition does not exist as a provision in relation to any other criminal offence in NSW, the letter states, and would allow a diagnosis of depression or anxiety to be used as a defence for failing to gain consent.
Hayley Foster, chief executive of Rape and Domestic Violence Services Australia, told Guardian Australia that there had been an increase in the accused in family violence cases using mental illness to defend their behaviour, a trend which she feared would spread to rape cases should the laws pass unamended.
Foster emphasised that she believed mental health should be considered when determining culpability in criminal offences, but that the proposed bill would give offenders a strong chance of escaping conviction.
She said evidence had shown that the majority of rapists knew their victims, and committed their offences as part of a pattern of behaviour, not because they had been mentally unwell at the time of their most serious offence.
Foster said she had not heard directly back from Speakman but had discussed the amendments with some upper house MPs.
Speakman said in a statement to Guardian Australia that the mental and cognitive impairment exception was included in the bill to ensure that a person with an impairment is not unfairly disadvantaged by the expanded reasonable belief test. The test went beyond the threshold recommended by the Law Reform Commission.
The crimes legislation amendment (sexual consent reforms) bill passed the NSW lower house on 10 November, and will be debated in the upper house this week.
Under the bill, the accused must establish they had a cognitive or mental health impairment at the time of the alleged offence, and that the impairment was the cause of the accused not saying or doing anything to gain consent.
