The figure comes from a confidential government document, which also warns of the need to rein in rapidly increasing costs as more Canberra children move into out-of-home care.

On June 30, the Community Services Directorate halted disabilities and therapy services in response to the introduction of the NDIS.

In anticipation of this transition, and in response to other budget pressures, the ACT government commissioned a $205,000 "sustainability review" from Ernst & Young in late 2016.

The terms of reference for this review, released under freedom of information laws, described the considerable impact the NDIS transition was having on the directorate's budget.

"The most significant transition has been Disability ACT exiting service delivery as the result of the agreement between Australian governments to establish the NDIS," the document read.

"This transition out of service delivery has been a three-year project and, put simply, has seen 350 staff exit service delivery."

Some of these staff had taken voluntary redundancies, and many had moved to new roles in the ACT government, a directorate spokesman said.

In addition to this loss of frontline staff, the directorate also had to pay $90 million to the Commonwealth government in 2017-18 to support the NDIS, the terms of reference showed.

When cash and other contributions were taken into account, this figure grew to $137 million in 2018-19, a directorate spokesman said.

The territory government provides almost 60 per cent of the funding for the NDIS in the ACT.

Cracks have begun appearing in the territory's disabilities services sector since the introduction of the NDIS.

Providers are struggling to run overnight respite services for high-needs children and people with psychosocial disabilities have had difficulties moving across to the new scheme.

Minister for Disability Rachel Stephen-Smith said in September the NDIS transition would ultimately improve disabilities services in the ACT.

"Throughout these, at times, uncomfortable changes, people have kept their eyes on the prize of a fairer, more equitable support scheme allowing people with disability in the ACT to have choice and control over how they live their lives," she said.

Ash van Dijk, the Community and Public Sector Union's acting regional secretary, said the transition had been "difficult and stressful" for impacted staff and clients.

"NDIS's takeover of services in the ACT impacted on hundreds of jobs," he said.

"This process wrapped up some time ago but we remain far from convinced that putting these services in the hands of private operators is in the best interests of staff or their clients.

"The CPSU worked to minimise the associated pain of this highly disruptive situation as much as possible."

The terms of reference document also detailed how recent reforms in youth protection services had been driven by a need to reduce costs in response to the growing number of children in out-of-home care.

"The initial modelling of this increasing demand highlighted the rapid increase in future costs," the terms of reference stated.

"As well as producing better outcomes for children and their families, the system aims to also constrain the predicted increase in future costs."

A spokesman for the Community Services Directorate said the government was "committed to the ongoing funding of the NDIS".

He added that early indications following reforms in child protection services were "promising", and that "the full benefits of early intervention will be seen over the longer term".

Contract details published by the ACT government showed that the initial cost of the Ernst & Young review was $150,000, although this blew out to $205,000.