Over the past 18 months, the tone has deteriorated amid speculation about cost blowouts, inadequate care plans for trial participants and qualifications of disability carers.

Freshly minted Minister for Social Services Scott Morrison's pre-Christmas announcement that welfare spending would have to be slashed to fund the insurance scheme, did nothing to assuage concerns.

Labor's spokeswoman on disability reform, Jenny Macklin, who oversaw the launch of the scheme in the Gillard government, was quick to dismiss the proposal as a "disgusting and cynical attempt" to justify a new round of savage welfare cuts.

"Any claim by the government that the NDIS isn't fully funded is completely wrong," she told Fairfax Media, pointing out revenue raised by the 0.5 per cent Medicare levy and the $6.5 billion saving in reforms to the private health insurance rebate. "The NDIS is fully funded into the future."

Although Morrison has not revealed much detail on how much funding is required and which areas of welfare will be cut as a result, Assistant Minister for Social Services Mitch Fifield weighed in with his view of the balance sheet.

Of the $22 billion cost of the scheme once fully rolled out in 2018-19, about $10 billion will come from the states. The Commonwealth will continue to contribute the $3 billion it currently spends on non-income disability supports plus $9 billion of new investment. According to Fifield, the Medicare levy will cover only about 40 per cent of the new investment – or about $3.6 billion a year –  leaving a significant gap.

The disability sector was quick to point out that the Medicare levy was never intended to cover the entire cost of the insurance scheme and that , rather than being a drain on the public purse, the scheme was a revenue raiser with a multitude of economic and social benefits.

Research commissioned by non-government disability peak body, National Disability Services, found that the scheme will increase the country's GDP by between $18-22 billion once it is fully rolled out, as people with disabilities and their family carers enter the workforce. It will also create jobs, with the Treasury estimating the number of disability care worker numbers will have to double to accommodate the needs of the 460,000 people covered under the scheme.

Fifield acknowledged the economic benefits of the scheme, as established by the Productivity Commission in its 2011 inquiry, but said they were a long way off.

"It's important to note that these economic gains won't be fully realised until the scheme reaches maturity, some 30 years after the scheme is fully rolled out at the end of this decade," he said.

This view overlooks the bigger picture, according to Dr Ken Baker, chief executive of National Disability Services.

"The federal government is focusing on fiscal pressure in the short term," he said. "In the long term, the economic benefits will outweigh the costs significantly."

Other disability advocates accuse the government of deliberately conflating welfare spending with the insurance scheme , despite the Productivity Commission's finding that the two should be kept separate.

"They are two different things," said Craig Wallace, president of People with Disability Australia. "One is about income replacement and the other is about providing specialist supports and services that people need to help them take part in work, volunteering and community activities. It's a bit misleading to say that you can organise trade offs between the two within budget."

Mary Mallett, the chief executive of the Disability Advocacy Network Australia, described it as cost shifting from one vulnerable group to another.

"They are using the NDIS as blackmail, saying we have to slash welfare so we can afford to pay for the NDIS," she said.

Along with contributing to revenue, the scheme is also expected to reduce costs in the welfare system as people become less reliant on income support and the health system as fewer people end up at crisis point.

Every Australian Counts national campaign director John Della Bosca warned that failing to fully implement the scheme would come at a very high price.

"The cost of the NDIS to the economy outweighs the cost of maintaining the current system," he said. "The NDIS is replacing a system that forces people with disabilities into high cost, inappropriate forms of substitute care in the health, mental health and criminal justice systems."

Chief executive officer of the Australian Federation of Disability Organisations Matthew Wright agrees, saying in the past people with disabilities and their families have been under-utilised at great expense.

"They have been locked away in institutions or nursing homes or in their own homes being cared for by family," he said.

"The NDIS will help people with disabilities into work, it will help their family carers return to the workforce. All those things will generate tax revenue but they will only happen with appropriate support which will come from the NDIS.

"People have very short memories. The NDIS may not be perfect but it's a lot better than what we have had in the past."

In numbers

• $22 billion – cost of the National Disability Insurance Scheme when it is fully rolled out in 2018-19.
• 460,000 people with disabilities to be covered nationally.
• $18-22 billion GDP increase once the scheme is fully rolled out.
• 70,000 extra disability care workers required.

Source: Federal Government, National Disability Services.