In her first appearance as Minister for Government Services and the NDIS at a Senate estimates hearing, Senator Reynolds signalled the government would continue with controversial reforms pushed by her predecessor, including "boundaries" on what supports are considered reasonable and necessary, and the introduction of independent assessments.

In a lengthy opening statement, Senator Reynolds focused on "sustainability challenges" facing the insurance scheme, pointing to more particpants with more expensive plans than first expected.

The NDIS is set to reach more than 530,000 participants next year, Senator Reynolds said, well over initial projections there would be 470,000 people enrolled when the program reached full rollout.

Under proposed legislation prepared by Senator Reynolds' predecessor Stuart Robert and leaked to the media, the government has been preparing to put in legislation a definition of "reasonable and necessary" supports that would be covered under the scheme.

"No two people have the same view of what reasonable and necessary actually is, and I think and I believe that this is something we must now address, having some commonality of understanding and application of reasonable and necessary," Senator Reynolds said.

The government had to balance the competing pressures of ensuring the scheme was fair and consistent and was sustainable financially, she said.

While Senator Reynolds started in the role by calling a pause on the wider introduction of controversial independent assessments instigated by Mr Robert, she confirmed on Monday the plan would go ahead in some form.

"I have at no time said that we won't proceed with independent assessments in some form," she said.

"It is imperative for all of the reasons we've said that we go ahead with some form of functional independent assessment."

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The assessments were necessary to ensure equal access to the scheme, because lower socio-economic Australians were on average getting NDIS plans with lower budgets than those of higher socio-economic status.

"What form they take is very much the subject of consultation," Senator Reynolds said.

Chief executive of the agency Martin Hoffman confirmed a team of "half a dozen or so" staff had been appointed to a "sustainability action taskforce" dedicated to finding ways to cut costs.

Mr Hoffman denied the taskforce was a "razor gang" and said "sustainability" was mentioned 14 times in the Act governing the scheme, and managing costs was part of the agency's ongoing responsibility.

Labor's spokesman on the NDIS Bill Shorten said he didn't trust the government to run the scheme.

"This government doesn't fundamentally believe in the National Disability Insurance Scheme, it thinks it's too generous, it thinks too many people with autism are getting funded," he said.

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