In a speech to be delivered at a major disability conference on Thursday, former NDIA board member John Walsh will lament that the scheme had not been implemented as its designers had envisaged.

"Australian Governments, collectively, have let down people with disabilities," Mr Walsh said ahead of his address to the Where To From Here? conference.

"The combination of arrogance, incompetence and power that the Commonwealth government brought to this - it's a pretty nasty combination.

"I don't think the NDIS, as envisaged has ever been implemented, so the data isn't going to show what it could really achieve. I just look at it as a lost opportunity."

Mr Walsh was one of the few prominent backers of the federal government's now-abandoned plan to introduce independent assessments for NDIS participants, believing it would create a fairer and more consistent system.

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He has also argued that rather than applying the "reasonable and necessary" test to every single participant support, it should be applied across their package as a whole.

In a submission to the parliamentary inquiry into independent assessments, Mr Walsh warned that continuing to allow debate over the merits of each support invited a "world of very significant cost escalation, and inevitable scheme crisis at some point".

The Morrison government has in recent months been warning of major cost blowouts, with the latest projections forecasting budget overruns in the order of $22 billion across the next four years. Mr Walsh this week expressed fears that in the absence of a more equitable system for allowing entry to the scheme and allocating funding, the government would "strangle approvals so that it will become a much more difficult scheme to negotiate".

The conference, which is hosted by Disability Services Consulting, will also hear speeches from NDIS Minister Linda Reynolds and former NDIS chairman Bruce Bonyhady on Thursday.

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