The National Disability Insurance Agency has published the modelling after Prime Minister Scott Morrison and NDIS Minister Linda Reynolds were this week accused of running a "scare campaign" for claiming the scheme's cost was growing beyond any previous prediction.

This week's federal budget revealed a drastic revision in the federal government's projections for participant costs, which are now expected to approach $30 billion in 2023-24.

That's about $5 billion more than what had been projected in October's federal budget.

The scheme's cost is forecast to reach $31.8 billion in 2024-25.

Disability groups, Labor and the Greens were quick to point out that forecast was largely in line with modelling in the Productivity Commission's landmark 2017 report, which predicted participant supports would cost taxpayers $30.6 billion in 2024-25.

Labor's NDIS spokesman Bill Shorten accused the government of "catastrophising about scheme sustainability and hyping fictional cost blowouts", while the Greens' Jordon Steele-John suggested the shift in rhetoric was part of a political plot to ram through its controversial independent assessment proposal.

After days of commentary, the NDIA published previously hidden modelling which showed participant costs were on track to exceed the Productivity Commission's estimate for 2024-25 by 12 per cent - or $3.4 billion.

It said the $30.6 billion figure included in the commission's report included $2.1 billion towards its operational costs.

The cost of participant supports was expected to be $28.5 billion.

The commission's predictions were based on figures supplied by the agency.

The release of the modelling is unlikely to assuage the government's critics, who will question why the figures are only now being released.

The decision also highlights the sensitivity within the agency and government ranks to external criticism.

The agency has twice in recent weeks published lengthy statements online defending itself following criticism from witnesses at the parliamentary inquiry examining independent assessments.

In a statement to The Canberra Times, a spokeswoman for Senator Reynolds reiterated the government's commitment to the NDIS.

She noted the Commonwealth's contribution to the scheme, which is jointly funded with the states and territories, would steadily rise to beyond 60 per cent by 2024-25.

"The Morrison government has always said it is committed to funding the NDIS as a demand-driven scheme, and this is reflected in more than $17 billion in additional Commonwealth funding in the last two budgets," she said.

"The budget for NDIS reasonable and necessary supports has been growing and will continue to do so in the coming years.

"However, we need to ensure we realise a scheme that can financially endure for many generations to come.

"I am committed to working with participants, the sector and state and territory governments in order to secure the future of the NDIS."

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