Between September and December 2017, 139 people joined the NDIS in the ACT but 101 others left. Figures provided by the National Disability Insurance Agency, which runs the scheme, confirmed 381 Canberrans had exited the NDIS since 2013.

When compared to those joining, the rate of ACT exits topped the nation, at three times the national average.

On Tuesday, the ACT's Disability Reference Group told an ACT parliamentary inquiry into the scheme that this high proportion of exits, many of whom were children, required an explanation, especially given the rate of locals joining the NDIS also appeared to be slowing down.

In 2016, the territory became the first jursidiction in Australia to be completely in the scheme, and a number of participants, advocates and service providers have told the committee their experience has dramatically declined ever since.

Some shared stories of those hospitalised by the stress of "proving their disability" or else left on hold to call centres for hours on end. Others had been dragged through the court system to appeal funding cuts and a number had bowed out of the scheme altogether.

A review of participants requiring early intervention resulted in "a high number of children exiting the scheme" in 2017, the agency said in its latest quarterly report.

But the committee heard on Tuesday that many of those children were still not getting crucial interventions prescribed by doctors, as delays in some cases stretched beyond treatment timeframes.

Gay Von Ess, who has worked with children who have autism for decades, said children under six were missing out under the new scheme as educational and group programs lost funding.

'They are being set up to fail ... what happens to that child when they hit school?" she said.

Dougie Herd, co-chair of the reference group which advises the ACT government, said a lot of problems raised by participants and providers he had consulted seemed to come back to simple operational decisions that could be remedied.

Like almost all the 70 organisations and individuals who made submissions to the inquiry, Mr Herd said he believed in the scheme. He came to Canberra to help with its roll-out, in a previous life working for the agency, and praised the work of the NDIA so far as "heroic" considering the speed of the roll-out.

It was unreasonable to expect the agency to bring almost half a million people into the scheme nation-wide within just six years of operation when its staffing levels were also capped, he said.

But that did not mean people should be left in limbo for months on end or prevented from contacting their planner directly, rerouted instead endlessly through call centres, he said.

Mental Health Community Coalition ACT said, while some people's lives had changed for the better since the roll-out of the NDIS, "many others appear to have been left in a worse position", as the sector was brought under "enormous strain".

"For too long now, when issues are raised about the NDIS framework, the [legislation] is used by government and the NDIA as the reason it just has to be that way," its submission said.

A report by the Commonwealth Ombudsman released on Tuesday revealed a back-log of more than 8,000 people were still waiting to have their support plan reviewed as of February. Some had been waiting more than nine months and the agency recieves about 620 new requests for review each week, the report said.

Commwealth Ombudsman Michael Manthorpe acknowledged the "considerable pressure" the NDIA was under to meet its targets but stressed this "must not be used as a reason to deprioritise or delay other work".

On Friday, Dr Sue Olney of UNSW told the committee her research on the NDIS suggested the speed of the national roll-out was not keeping pace with the reality of a market still in development, leaving some people behind.

Leith Felton-Taylor of MHCC ACT said the NDIS was increasingly replacing the mental health system in the ACT, as previous territory programs shut down, and that had opened up a dangerous gap for vulnerable people not in the scheme.

While the agency is currently piloting a new improved pathway program in Victoria, most of the issues raised had been a problem for a number of years and reform was now a matter of urgency, the committee heard.

The agency was unable to respond to further requests for comment before deadline.