Delayed diagnoses, misdiagnoses and deaths possibly linked to inadequate treatment are among issues being investigated by the royal commission during a public hearing in Sydney that begins on Tuesday.
Chair Ronald Sackville QC said the inquiry will hear directly from people with cognitive disability about their experiences with the health system.
"We're also going to hear from a number of parents of people with cognitive disability about their experiences, and that includes unhappily some parents who have lost children with cognitive disability," Mr Sackville told AAP on Monday.
"I think you will find over the next two weeks that there is evidence that should disturb all Australians and should make all Australians aware that there are very, very serious problems that need to be addressed."
The inquiry will examine the extent to which people with intellectual disability, autism and acquired brain injury disproportionately experience significant health problems, including higher mortality rates.
"We'll hear about delayed diagnoses/misdiagnoses of people with cognitive disability, and of course that links into the necessity for better training of medical and other health professionals in dealing with the health problems of people with cognitive disability," Mr Sackville said.
"Those health problems are often exceedingly complex," he added.
One issue is whether people with cognitive disability have been understood when they tried to explain their health difficulties.
"One of the themes that emerges from the evidence is the feeling among people with cognitive disability or their families that they've not always been listened to, and this can lead to misdiagnosis or a failure to diagnose health problems that can of course sometimes be very serious," Mr Sackville said.
"There is certainly a view among people who have tragically had children who have passed away that the way in which they were treated within the health system was not as they were entitled to be treated or should have been treated."
UNSW research found deaths due to potentially avoidable causes were "alarmingly high" in people with intellectual disability, who faced major barriers to receiving appropriate and effective health care.
A royal commission issues paper also raised questions about the possible over-prescription of medication to people with cognitive disability and cases of health professionals attributing symptoms to a person's disability rather than investigating a specific health issue.