A bill currently before Parliament makes only cosmetic changes, despite moves overseas and throughout Australia to radically change involuntary treatment and abandon the idea that people should be assessed primarily on the risk they pose to the community, Greens MP John Kaye said.

He feared another a tragedy could occur, such as when Sydney man Anthony Waterlow killed his father and sister after he was not given involuntary treatment.

However, the government says the risk assessment model has broad support among academics and carer groups.

Christopher Ryan, a psychiatrist at the University of Sydney's centre for values, ethics and the law in medicine, said the bill contained some good changes but was "quite disappointing".

"Mentally ill people who understand the information about being treated, and come to a decision not to have treatment, can still have treatment forced on them if they are deemed at risk," he said. "Imagine if an oncologist was allowed to force chemotherapy on a patient.

"We could be looking at the best part of a decade before NSW comes into line with other parts of Australia."

There were more than 5400 orders for involuntary treatment made in NSW in 2012 to 2013, according the Mental Health Review Tribunal.

People can be treated involuntarily if they are identified during a medical assessment as posing a risk to themselves or the community. However, the risk assessments are notoriously unreliable.

Western Australia, Victoria, Tasmania and South Australia have all changed or are in the process of changing their laws to add assessments of a person's capacity to agree to treatment.

The chief executive of Mental Health Carers ARAFMI NSW, Jonathan Harms, said while there had been extensive consultation on the act, many of the changes that needed to be made would have cost a lot of money.

"Mental health resources in NSW need to double in size, and until then we are not going to get the system we deserve," he said.

Sascha Callaghan, also from the centre for values and ethics in the law at the University of Sydney, said under the UN Convention of the Rights of People with Disabilities, Australia was obliged to move towards a system that better respected people's rights.

"An opportunity to modernise laws that have been in place since the 1880s has been missed," Dr Callaghan said. "It does violate the rights of people with a mental illness because it still treats them on an unequal basis.

"People often experience the system as being debasing and violative."

Dr Kaye said: "Mentally ill people will see very little change to their ability to receive treatment or their rights to have a say in how they are medicated."

"Nothing has been done to address the failure of the legislated processes that resulted in the Waterlow tragedy."

But Mental Health Minister Jai Rowell said the "vast majority" of academics, carer groups and patients supported the risk assessment model.

"In some cases, the risk to the person and/or the wider community outweighs the patient's consent," he said.

He said the changes were consistent with Australia's UN obligations, as the legislation says consent to treatment should be sought wherever possible.

"In instances where it is not possible for the consumer to provide consent, nor be supported to provide consent, decisions regarding treatment may still be made after considering the views and preferences of the consumer," Mr Rowell said.