The treasurer has urged state and territory counterparts to get on board with looming NDIS changes, as premiers say they will not be able to afford dealing with the reforms.

As part of measures to reduce spending on the NDIS, about 160,000 people will be moved off the scheme by 2030 and into state-run support systems.

While premiers and chief ministers have criticised the measures, Jim Chalmers has written to states and territory treasurers saying the reforms were necessary and would save hundreds of millions of dollars from their bottom lines.

"We all have a stake in reforming the NDIS so it can provide the level of care that people need and for our governments to be able to sustainably fund the scheme," Dr Chalmers wrote in the letter, sent on Thursday.

"We have a shared interest in achieving sustainability and securing the future of the scheme as well as its social licence with the broader public, and we are committed to working with you on the implementation of these reforms."

Figures demonstrate states would save more than $2.8 billion combined over the next four years from the NDIS changes.

NSW will save the most across that time period, with $890 million, followed by $720 million in savings for Victoria, $580 million for Queensland and $300 million for WA.

Dr Chalmers said the states previously agreed to increase their contribution to the NDIS in return for sharing in $25 billion in federal funding for public hospitals.

"States and territories agreed earlier this year to increase their NDIS contribution escalation rates to be in line with actual scheme growth," he said.

The federal treasurer said the NDIS still costed too much compared with other social programs and was growing too fast.

But NSW Premier Chris Minns said the state could still not afford to ramp up NDIS supports to offset the federal changes.

"We have to be really frank with people, we can't offer at the state level the kinds of services that are being rolled out at the NDIS," he told ABC TV.

"The reason for that is not because we're stingy, or we don't want to do it, but we're full on and flat out providing emergency care at our hospitals and education and building houses and all the things that we're obligated to do at the state level.

"I don't want to make the same mistakes that were initiated over 10 years ago when the NDIS was created."

David Cullen, top economic advisor for the NDIS from 2016 to 2022, has urged recalcitrant premiers to get on board with major reforms.

He 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."

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.

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 NDIS, and said new support systems would need to be built.

Autistic people shouldn't be "the shock absorbers" for systems that government had not fixed and good reform was not about what changed, but the order in which it was implemented, she said.