Students with disabilities could miss out on hundreds of millions of dollars in extra funding because of serious concerns about the reliability of a landmark national audit into the number of school children with special needs.

Disability advocates fear the questionable long-awaited study, quietly released on Tuesday, will be used as an "escape clause" for governments to again delay the delivery of extra funding for students with disabilities.

The Nationally Consistent Collection of Data on School Students with Disability (NCCD) shows 18 per cent of school children in 2015 had a disability while only 7.4 per cent received targeted funding.

The study shows 20.7 per cent of students in Queensland have a disability, far higher than 11.3 per cent of students in Tasmania and 13.6 per cent in the Northern Territory. It found 17.9 per cent of NSW students and 17.1 per cent of Victoria students have a disability.