Spending on the NDIS fell potentially $1 billion short of forecasts in the past financial year, internal documents which have prompted the Labor government to accuse its predecessors of overestimating the cost of the scheme have revealed.
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The Greens said the budget underspend meant "not a single cent" could be stripped from the scheme in the October budget.
The government has already used some of the funds to help providers cover overhead costs, reducing the "surplus" to about $700 million.
The cost of the NDIS has been at the centre of a political storm in recent years, as the Morrison government repeatedly warned the scheme was fast becoming unsustainable as participant numbers surged beyond expectations.
The Coalition's final budget in March allocated a massive $157 billion to the scheme through to 2024-25, including $29.2 billion in 2021-22, to cover the massive growth.
The funding injection was based on the latest projections about costs and participant numbers from the scheme actuary.
An incoming minister brief, which the agency prepared for new NDIS Minister Bill Shorten, showed participant numbers had outstripped the actuary's expectations for the past financial year.
But average payments to participants fell short of projections, resulting in an underspend of potentially $1 billion - or 3 per cent of overall scheme expenses.
The Coalition made the controversial decision in 2019 to use a $4.6 billion underspend on the NDIS to help return the budget to surplus, before the COVID-19 pandemic struck and wreaked the government's finances.
Greens senator Jordon Steele-John said the Albanese government must not follow suit, urging it to ensure the unused funds were reinvested into the scheme.
"My immediate reaction [to revelations of the underspend] was that there is now no argument whatsoever for taking a single cent out of the NDIS," he told The Canberra Times.
"It [the scheme] is running $1 billion underspend and that money must be kept in the scheme for the future, when more resources might be required."
Details of the budget underspend were included in the same briefing document which revealed the National Disability Insurance's agency's budget was "inadequate" to support the scheme's massive growth.
Mr Shorten first foreshadowed the budget underspend in June, expecting it would be "in the order of hundreds of millions of dollars". The incoming government brief reveals the figure is set to be higher than that.
In response to questions from The Canberra Times, Mr Shorten said the former Coalition government had "overestimated" the cost of the NDIS.
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The new minister didn't respond directly when asked if the funds would be reinvested into the scheme, or allocated elsewhere.
However, Mr Shorten noted the NDIA board had made a decision earlier this year to increase prices for participant supports delivered by support workers. More than $500 million had also been allocated to help providers cover overhead costs which hadn't been accounted for in the past.
That had reduced the surplus to $700 million, Mr Shorten said.