Every Australian Counts director Kirsten Deane said problems with the scheme were to blame for what the government called a lack of demand and slow uptake.

Ms Deane said she hoped to have complaints and solutions raised on Wednesday to NDIS Minister Stuart Robert.

Treasurer Josh Frydenberg revealed last week the Australian government was only $700 million in deficit and on the brink of a surplus, a $13.8 billion improvement on the deficit predicted in last year's budget

But lower-than-expected spending on the scheme was partially to thank for the good news.

Ms Deane said since the underspend was revealed, her advocacy group had received more than 500 messages from stakeholders.

She said complex red tape was hampering people's efforts to sign up and the government was behind schedule in bringing people on to the scheme.

People could be waiting up to two years for a wheelchair if they applied for one under the scheme, she said.

"That's not an underspend, that's you desperately needing a wheelchair," Ms Deane said.

"You can't work your way through the system to get one in a timely matter."

She said she'd encountered people who hadn't spent their funding because they didn't know who to approach or how to navigate the scheme.

"So our really clear message is yes, there is an underspend in the NDIS but that is proof that the scheme is not working the way that it should," Ms Deane said.

Australian Associated Press