David Cullen, the National Disability Insurance Scheme's top economic advisor from 2016 to 2022, has urged recalcitrant premiers to get on board with major reforms.
About 160,000 people will be moved off the program and onto separate state-run support systems.
Premiers have reacted coolly to the proposal, claiming it will simply shift costs from the federal budget onto their own.
But Professor Cullen - now a partner at Evaluate Consulting - said the massive overhaul would leave states better-off in the long run.
"State premiers are failing to understand that if it doesn't change, the NDIS is going to cost them more and more as the years go by," he told AAP.
"The cost of changing the NDIS is less for them than the cost of leaving it as it is."
Under a funding agreement struck in January, premiers are required to increase their spending on the scheme by eight per cent each year, unless the growth rate of the NDIS falls below that figure.
In exchange, the federal government promised to provide the states an extra $25 billion in hospital funding.
While Prof Cullen said offering more disability supports outside the NDIS would inevitably cost state coffers some money, he argued it would be less than if the scheme continued on its current trajectory.
"It's in the interests of the states to build those supports," he said.
Health Minister Mark Butler's changes are expected to reduce the cost of the scheme from a projected annual figure of $70 billion by the end of the decade, to about $55 billion.
Some of that $15 billion saving will be borne by the states to support people no longer on the scheme, but Mr Butler argues it will be cheaper to help people with mild to moderate disabilities outside the NDIS.
But many disability advocates are concerned about the support people will be offered once they're moved off the scheme.
Australian Autism Alliance co-chair Jenny Karavolos warned people with autism often slipped through the cracks before the advent of the NDIS, and said new support systems would need to be built.
"Autistic people shouldn't be the shock absorbers of systems government has not yet fixed," she told AAP.
"Good reform is not just about what changes - it's the order in which it's implemented ... we need to make sure that those systems are ready."
The government has already flagged plans to move children with mild to moderate autism off the NDIS under a program called Thriving Kids.
Ms Karavolos said she expected adults with similar conditions to be shifted onto other supports under Mr Butler's changes.
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