Jeffrey Smart, who has Parkinson's disease, told a parliamentary committee investigating the National Disability Insurance Scheme his independent assessment was inaccurate, incomplete and irrelevant.

He said the report would not be useful to determine eligibility or levels of support.

It was done by a clinical psychologist who had little detailed knowledge of the condition, had not met him before and, crucially, did not have access to his degenerative disease history.

Mr Smart said he was asked to make a cup of tea and peel a potato, and was questioned about intellectual disability and communication skills as if he had a brain injury.

He said his application was rejected because of ignorance about the importance of early intervention.

"I was offended and felt belittled," he said.

"It is a complex disease that presents differently in each person."

More than 20 per cent of people diagnosed are still of working age.

More than a tremor or loss of movement, many report loss of smell, anxiety, depression and fatigue, facial masking, joint stiffness, a soft voice, and the loss of fine motor skills.

Committee chair Kevin Andrews, a former social services minister, said Mr Smart's evidence was important in understanding how independent assessments were being experienced during trials, ahead of a broader rollout.

Quadriplegic and disability advocate Dougie Herd was shocked NDIS Minister Linda Reynolds said the scheme relied too much on "natural empathy" and individual judgments.

"There was not a scintilla of empathy bias on the part of the doctor that gave me my form and that's not just me being a smartypants," he said.

"I read and listened to what the minister said about empathy the other day. If I was still a public servant I would be shocked to hear a minister say what she said."

At a previous hearing, Senator Reynolds warned the committee the NDIS was on an "unsustainable growth trajectory".

Despite community concern, she still wants to introduce independent assessments of the level of support needed, perhaps every three or five years.

Annual plans are currently drawn up by the scheme's planners with input from a person's own medical team.

"We're relying, I think, too much on individual public servants' judgement and also their natural empathy," Senator Reynolds said.

The federal budget showed the cost of NDIS participant plans growing to $31.9 billion by 2024/25.

Australian Associated Press