As the government prepares to introduce sweeping eligibility changes to the $56 billion program, autism advocates say it's not clear how long it will take to build state-run supports for people who will be kicked off.
"The systems aren't ready. If the systems aren't ready for reform, then there should be no transition," Australian Autism Alliance co-chair Jenny Karavolos told AAP.
"People will be without support. People will leave their jobs. Carers who are supporting people will need to take a bigger load... there's increased mental health costs, increased family stress, increased future support needs."
The federal government hopes to curb spiralling costs by moving thousands of people with more mild and moderate disabilities onto other supports run by the states.
But some states have pushed back, with the strongest criticism coming from Queensland where the Liberal-National government is refusing to play ball, while other states have claimed the changes are an unfair hit to their finances.
The reforms, which the government hopes to pass through parliament in the coming weeks, are currently being examined by a snap three-day inquiry.
But their passage into law is not guaranteed. The Greens are fiercely opposed and while the coalition backs the principle of the changes, it is threatening to hold the bill hostage to gain political leverage on unrelated reforms to tax on investments.
Under the current plan, some participants, including children with mild to moderate autism will begin being shifted off the scheme from January 2028.
But with new support coordination not expected to begin until July of that year, Ms Karavolos said the government should consider a delay of at least six months.
"How are we (undertaking) a mass move of people into a system, before the core elements that help navigate and support you are in place?" she said.
The Australian Autism Alliance is one of a number of groups scheduled to testify on Wednesday, day two of a hearing into the proposed changes.
Government agencies including Services Australia and the NDIA - the body responsible for administering the scheme - will also give evidence, along with federal police and the Australian Criminal Intelligence Commission who are expected to be asked about reports of rampant fraud in the NDIS.
The opposition has criticised the length of the inquiry, calling for more scrutiny of the reforms.
