Feedback from 145 people in the disability support scheme is being used to shape a major change of the system.
Independent assessments by National Disability Insurance Agency-approved providers will be mandatory from next year for people not only accessing the scheme but already part of it.
A survey found 91 per cent of people who took part in an independent assessments pilot were satisfied with the experience, but it was revealed in a Senate estimates hearing only 145 people provided their opinion.
Greens senator Jordon Steele-John was appalled changes would be built from such a small sample, saying they were already causing concern in the community.
"This is a significant change that has caused a lot of people a massive amount of distress," he said on Thursday.
"That's outrageously unacceptable."
The assessments are to determine the level of support an applicant needs, and will replace ones normally done by a health professional of the participant's choosing.
NDIS actuary Sarah Johnson said the 145 respondents of the 500 pilot participants - all in NSW - was a "good sample size".
A second pilot is due to begin later this year.
With more than 400,000 people on the NDIS, the government expects it to take a few years before all existing participants go through an independent assessment.
Senators were also told the scheme's legal bill has risen to $29 million, with about half spent on decision review cases.
NDIA boss Martin Hoffman said the body had spent $18.4 million on external legal fees in the 2018/19 financial year.
That rose to $29 million the period after, with Mr Hoffman telling senators there had been an increase in people joining the scheme.
More than $13 million was spent on cases in the Administrative Appeals Tribunal, up from $9.4 million in 2018/19.
People can ask the AAT to review decisions made by the NDIA for a variety of reasons, including being part of the scheme.
Disability support plans can be reviewed, as can decisions on who is deemed to have parental responsibility for a child.
Senators were also told there are 21 investigators at the NDIS Quality and Safeguards Commission, with the agency set to hire five or six more.
Australian Associated Press