A fight over whether mental health conditions should mean people don't have to take active steps to seek consent is the final hurdle for a historic overhaul of sexual assault laws in NSW.

The draft law creates a requirement of 'affirmative consent' - which means that people have to say or do something to find out whether their partner consents to a sexual activity, or they could be guilty of sexual assault.

Frontline organisation, Rape & Domestic Violence Services Australia, has criticised an exception in the bill for people suffering from mental ill health or a cognitive impairment, warning that it's too broad.

The bill says that the affirmative consent requirement doesn't apply if a person accused of sexual assault was suffering from a cognitive or mental health impairment at the time, and that was a cause of the failure to get consent.

They'd still have to believe the other person was consenting.

The section will be used "as a loophole for accused persons to avoid the requirement to take steps to establish consent" if the bill is passed, CEO Hayley Foster wrote to MPs this week. Other organisations, including Domestic Violence NSW, ACON and the Older Women's Network, have signed on to the letter too.

Ms Foster told AAP she was surprised to see the bill didn't require an accused to prove they had a "substantial cognitive impairment" to fall within the exception.

There are already protections in the criminal justice system for people with mental health problems, she said.

"There shouldn't be additional protections for people who are accused of sexual assault than (there are) for other crimes," Ms Foster said.

Attorney-General Mark Speakman on Thursday evening said the definitions had been developed with forensic mental health experts and were consistent with other criminal laws.

"This is not a 'get out of jail free' card - an offender with a cognitive impairment or mental health impairment can still be convicted if all elements of the case are established beyond reasonable doubt," Mr Speakman said.

Labor, the Greens and One Nation are pushing for changes to the exception, which will be debated in the upper house on Friday.

A Greens amendment would tweak the bill to provide that the impairment must be "the cause" - rather than "a cause" - of the failure to seek consent. Labor wants it be "a substantial cause".

If One Nation gets its way, a person would have to show they have a "substantial" cognitive impairment before they could fall within the exception.

Passage of the bill is all but assured, no matter how the debate on the amendments is resolved.

The historic reform, years in the making, has support from the government, Labor, the Greens and other minor parties.

It comes after Victoria's government last week announced it would introduce similar changes to that's states Crimes Act.