Seven in 10 new National Disability Insurance Scheme participants are under the age of 15, and the number of children covered is now almost double the original forecasts, prompting warnings that efforts to constrain the program’s growth are not ambitious enough.

Treasury forecasts the NDIS will cost the state and federal governments $50.8 billion next financial year, and the scheme is likely to overtake the age pension as the most expensive social program in the country within a decade. Spending on the NDIS will outstrip the defence budget in 2026-27.