Background: People with neurodevelopmental disabilities experience poorer health and substantially higher mortality than the general population. These outcomes reflect both disability-related conditions and the cumulative impact of social disadvantage and structural inequities. Quantifying mortality inequalities is essential for effective population health responses. Prior research largely assesses all-cause inequality on a relative scale; we estimate cause-specific mortality on both relative and absolute scales.

Methods: We conducted a retrospective cohort study using linked census, disability service, and mortality data for Australians aged 10-64 years. A time-varying approach accounted for delayed entry into disability services. Mortality rates (per 100,000 person-years) were calculated by sex, comparing those with neurodevelopmental disability to the general population. Rates were age-standardised to the disability population and analysed across 5- (all-cause) and 15-year (cause-specific) age groups. Rate ratios (RR) and rate differences (RD) were estimated with 95% confidence intervals from 1,000 bootstrap samples.

Results: Age-standardised mortality rates were 8.7(95% CI: 8.3–9.1) times higher in females with disability and 5.0(95% CI: 4.8–5.2) times higher in males. Absolute inequalities were substantially higher for both females (RD 650 per 100,000; 95% CI: 620–683) and males (RD 453 per 100,000; 95% CI: 430–475). The highest relative inequality was observed in females 20–24 years (RR 12.7; 95% CI: 10.2–15.6), and the greatest absolute inequality in females 60–64 years (RD 3,585 per 100,000; 95% CI: 3,137–4,043). Respiratory and neurological conditions drove the largest relative inequalities, whereas the highest absolute inequalities were in malignant neoplasms and cardiovascular diseases.

Conclusions: People with neurodevelopmental disabilities experience substantial mortality inequalities. High relative inequalities reflect disability-specific vulnerabilities, while large absolute inequalities in chronic diseases point to inequities in access to mainstream prevention and care. Addressing this requires integrated strategies spanning disability-specific supports and equitable access to public health and healthcare systems.