ACT MP David Garrett says the justice system is failing mentally ill people and must be changed.
A man, 69, who has Parkinson's disease and possible dementia, is on remand in Rimutaka prison while facing charges of indecent and common assault of a woman at a care home.
The man's lawyer says he's a prime candidate for a maximum prison term under the 'three strikes' legislation, if his health leads to more offending.
Mr Garrett was the MP who pushed for the 'three strikes' law. He says the man does not belong in prison and there are too many other people with mental health problems behind bars.
He says the system is not looking after people who are in what he calls "between the cracks".
Mr Garrett also says he does not believe any judge would give someone with a condition, like dementia, a maximum sentence.