The Morrison government has been accused of casting people with a disability as a financial "burden on society", as it continues to argue the rising costs of the NDIS needs to be contained to keep it sustainable.
One of the architects of the NDIS has also questioned the government's recent alarm about the scheme's financial position, given its own reports published over the past year had projected costs remaining largely in line with Productivity Commission estimates.
Mr Morrison said the scheme's budget was expected to exceed $26 billion next financial year, as costs rose "more than was ever contemplated or expected" by its designers.
The Productivity Commission had in 2017 estimated the NDIS would cost $22 billion once it was fully rolled out.
Mr Morrison said the biggest driver had been the increase in average payments per participant, which had risen almost 50 per cent between 2017 and 2020.
The government now expects the NDIS to support 523,000 people - almost 50,000 more than anticipated in the commission's 2017 report.
"So this has got much larger than the early vision intended," Mr Morrison said in a speech on Thursday.
"That vision was a good vision. It was a compassionate vision. It was a caring vision. But to remain caring, you've got to make it work and you've got to make it sustainable and you've got to ensure that it can keep delivering.
"You'll hear plenty of others who will argue that the sky should be the limit but we all know that that is not a realistic objective.
"To ensure the NDIS is here for generations of Australians to come, I intend to protect it. I intend to sustain it. I intend to exercise my responsibility to manage those increased cost pressures."
The manner in which the government has framed the scheme's financial position has drawn sharp criticism from experts in the sector.
Professor Gemma Carey, who researches NDIS policy at UNSW's Centre for Social Impact, said the Morrison government had been running "a few misleading lines".
Prof Carey said the government's focus on cost created the impression that the scheme and its participants were a financial "burden on society".
"These are essential services, this is what governments are supposed to do [fund the NDIS]," she said.
"When you present them as a burden, you are only showing half the balance sheet. You're not showing the benefits and cost savings."
Professor Carey said the government's proposed changes, in particular independent assessments, would set the NDIS "back to somewhere near, or worse" the "inequitable and fragmented system" disability support system which existed before the scheme was introduced.
Bruce Bonyhady, the inaugural chairman of the National Disability Insurance Agency, said it was impossible to assess the NDIS's sustainability while crucial documents - specifically the annual financial report produced by the scheme's actuary - remained hidden from the public.
Professor Bonyhady, who also chaired the advisory panel which informed the Productivity Commission's work to design the NDIS, called into question the government's rhetoric around the scheme's financial position.
The agency's latest annual report highlighted the various pressures facing the scheme, but stated that costs projections for 2021-22 remained in line with what had been estimated in the commission's 2017 report.
"Is it that the assumptions that have changed, or is it some actual experience that has changed in a short period of time that has led to this?" Prof Bonyhady said.
"At this point it is not clear what led to this change.
"If we are going to have a debate about the sustainability of the scheme then all the information that pertains to sustainability has to be put on the table. This scheme is far too important to be the subject of party politics."
Our journalists work hard to provide local, up-to-date news to the community. This is how you can continue to access our trusted content: