Professor Bruce Bonyhady, chair of the National Disability Insurance Agency and economist says spending on the NDIS is expected to run $5 billion under budget by the end of the financial year.
His comments come after it was revealed on Tuesday that spending for the scheme came $10 billion under budget in the last financial year.
The Treasury attributes the figure to lower numbers of participants joining the scheme and less uptake of individual packages than anticipated.
But Bonyhady says it is not a good sign that vulnerable people are not making full use of entitlements, as participants were, on average, spending only 70% of funding allocated to them.