"The Department of Corrections needs to recognise that the courts are sending more and more people to prison for longer ... and we are going to have an aging prison population."
Her client, Donald Liddington, 75, is serving preventive detention for raping young girls in the 1980s and 1990s. He is considered too ill for prison but too dangerous for the community.
The Mental Health Foundation's Chief Executive, Judi Clements, said she was surprised the service did not already exist.
"For some time there's been some concern about the prison service generally and how it can respond to the mental health needs of the inmates.
"The fact that there's an aging prison population means it's something that the corrections service needs to plan for."