The National Disability Insurance Agency's spend on external contractors and consultants has grown faster than its budget for permanent staff, ballooning to $166 million last year.
The spend on permanent staff has also not kept pace with an explosion in number of participants joining the scheme, analysis has shown.
Like other arms of the public service, the National Disability Insurance Agency has a ceiling on the number of permanent staff it can employ.
As of June 2018, it had 2634 direct staff, 1799 contractors and secondees and 3439 so-called "partners in the community" (people who work for community organisations which provide NDIS services).
Labor took aim at the average staffing level cap on Thursday, after Treasurer Josh Frydenberg unveiled a $4.6 billion under-spend on the NDIS in the final budget outcome for 2018.
Finance Minister Mathias Cormann suggested the savings were due to lower than expected demand, as states and territories "came on board more slowly than we would have liked".
But Treasury spokesman Jim Chalmers said the "absurd" cap meant "they can't get the packages out the door".
Government services spokesman Bill Shorten said the NDIS was "constipated".
"The reality is the demand is there," Mr Shorten said.
"The money is meant to be there and getting out to the people."
Contractor spend booms
An analysis of the agency's past five annual reports reveals while it spent more each year on permanent staff than contractors, the budget for permanent employees lagged in terms of year-on-year growth.
In 2013-14, when there were fewer than 9000 participants in the scheme, the agency spent $47 million on staff and $16 million on consultants.
In 2017-18, that grew to $235 million on staff and $166 million on contractors.
The number of participants had grown by more than 2000 per cent in that time, to about 180,000.
But while growth in participant numbers and contractor spending was neck-in-neck most years, it was significantly lower for staff spending.
The explanations varied year to year. In 2014-15, it said employee spend was $33.1 million below budget while supplier expenses $27.7 million higher as "the agency engaged more contractors than expected ... as a result of delays in recruitment".
A year later - ahead of the full scheme launch - the agency was forced to adopt a service delivery operating model, which effectively reduced its average staffing level cap and use "non-government outsourced models to achieve the same outcome".
"This was part of a broader strategy by the government to control growth in public sector staffing levels," the report said.
Advocates blame cap for delays
The debate over the cap comes as a parliamentary inquiry receives more evidence about the problems stemming from it.
ACT Disability Aged and Carer Advocacy Service chief executive Michael Bleasdale urged the committee to urgently repeal the staffing restrictions.
"ADACAS advocates advise that there are sometimes very significant delays before a planning meeting is made available - and that the most commonly quoted reason for delay is a lack of planners available," Mr Bleasdale said.
Allied Health Professions Australia told the committee the caps "almost certainly" was causing problems with the NDIS planning process.
"We are aware of long delays experienced by many participants both at the planning stage, and at the plan review stage. This suggests that there may be staffing issues impacting the planning process," its submission said.
Australian Lawyers Alliance president Andrew Christopoulos also said his members and clients had experienced significant delays in getting plans finalised.
"This suggests that there are not enough planners to meet demand," Mr Christopoulos said.
Mr Christopoulos also told the committee the background and experience of planners appeared to be "variable".
"Anecdotally, we have been told that a participant's experience with the NDIS is largely defined by their planner - if you get a 'good' one, you are likely to have a positive experience," Mr Christopoulos said.
"However, if you get a 'bad' one, your plan is unlikely to be sufficient nor appropriate, forcing you down the path of internal and external review.
"[This] results in a significant administrative burden and financial cost for the NDIS because it increases the numbers of complaints and requests for internal review."
A spokeswoman for National Disability Insurance Scheme Minister Stuart Robert last week refused to say whether the cap would be reconsidered.