Budget estimates are expected to show the NDIS costing more than $30 billion in 2024-25, almost $10 billion more than previous estimates.

Tuesday night's federal budget will contain additional funding for the disability support scheme.

But the government is looking at ways to make the NDIS more affordable.

"We need to ensure that it continues to remain sustainable," Treasurer Josh Frydenberg told reporters.

"But under the coalition, we are deeply committed to the NDIS and we'll always ensure that it is fully funded."

The government is set firm on introducing independent assessments for all NDIS participants - despite widespread opposition - in order to cut costs.

Finance Minister Simon Birmingham said the program was growing faster than ever expected, with record numbers of participants joining the NDIS.

"That's why as a government we are honouring our commitment to fully fund it, managing the program for those record participants," Senator Birmingham said.

"But of course, we will manage it as carefully as possible to make sure it's sustainable for the future, too."

The NDIS supports 450,000 people but that is expected to grow to 530,000 within a few years.

The average payment per participant has also grown significantly, with the Commonwealth's share of the costs on an upward trajectory.

Australian Associated Press