Paralympian Dylan Alcott says he's "devastated" the Government is championing a balanced budget while also confirming a $4.6 billion underspend on the National Disability Insurance Scheme (NDIS).

Key points: Alcott is Australia's patron for International Day of People with Disability

Alcott is Australia's patron for International Day of People with Disability He tweeted that he is heartbroken for families "who try to get funding but can't"

He tweeted that he is heartbroken for families "who try to get funding but can't" Finance Minister Mathias Corman dismissed suggestions the funds should have been redirected into disability services

The nine-time wheelchair tennis major winner and disability advocate was speaking after Treasurer Josh Frydenberg released Treasury figures showing the budget deficit for the 2018-19 financial year was just $690 million, down from the $4.2 billion figure forecast in April.

"Pretty devastated to read today that Aus went into 'budget surplus' today due to $4.6b 'saved' on NDIS funding due to delays," Alcott wrote on Twitter.

"I see the heartbroken families of people who try and try to get funding but can't, robbing them to be independent, contributing members of society. Fix it."

Where the budget stands: The country ended the financial year with a deficit of $690 million

The country ended the financial year with a deficit of $690 million In April the EOFY deficit was predicted to be $4.2 billion

In April the EOFY deficit was predicted to be $4.2 billion Nominal GDP grew by 5.3 per cent over the financial year, or 1.9 per cent in real terms

Nominal GDP grew by 5.3 per cent over the financial year, or 1.9 per cent in real terms The labour market grew by 2.6 per cent across 2018-19

The labour market grew by 2.6 per cent across 2018-19 Net debt stands at $373.6 billion

Alcott's comments come after the Federal Opposition was quick to jump on the lowered spend on the NDIS, with Shadow Treasurer Jim Chalmers saying it was "propping up" the budget.

"The way they've got there is a combination of bad faith and good luck," he said.

Labor's NDIS spokesman, Bill Shorten, said: "It's a national shame, it's a disgrace."

"The fact of the matter is, the Morrison Government is balancing its books off the backs of Australians with disabilities and their loved ones."

Alcott has won gold for Australia in wheelchair basketball and nine wheelchair tennis grand slam singles titles.

He established the Dylan Alcott Foundation, "with the core purpose of helping young Australians with disabilities gain self-esteem and respect through sport and study" and was appointed Australia's patron for International Day of People with Disability.

The Government said spending on the NDIS had more than doubled in 2018-19, saying the underspend was caused by a slower-than-expected uptake of people into the program, which Finance Minister Mathias Cormann blamed on state governments.

Mr Cormann dismissed suggestions the Coalition should have redirected the funds into disability services.

"It makes absolutely no sense to put money aside when the money will be there," Senator Cormann said.

"Essentially, in any budget, at any budget update you have movements up and down. You have demand that is higher than expected and you have to pay more. Sometimes you have demand lower than expected.

"The situation with the NDIS is unique, because we are going for a massive transition.

"We are going through a transition from a state-based funding arrangement, where there was a lot of unmet demand and a lot of inadequate support for Australians with a disability, to a situation where every Australian who has a demand for disability services, will receive that support tailored to their needs."

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 26 seconds 26 s Shadow Treasurer Jim Chalmers says NDIS cuts are "propping up the budget"

Mr Frydenberg said there had been a $13.8 billion improvement in the underlying cash balance since the budget was released in 2018.

"These outcomes demonstrate the Government's economic plan is working and confirming the budget is on track and will be back in the black for the 2019-20 year," he said.

"Importantly, Australia has completed its 28th consecutive year of economic growth, and maintains its triple-A credit rating, with more than 1.4 million jobs created since we came to Government."