Recent evidence suggests the male predominance in Autism Spectrum Disorder (ASD) may be decreasing. Secondary analyses of Australian Medicare data (paediatrician/child psychiatrist items for diagnosing ASD before age 13) were used (N = 73,463 unique children from 1-July-2008 to 30-June-2016). Cumulative incidence of ASD in 4-year-olds in 2015/2016 was 1.10% [95% CI 1.06–1.14], males 1.66% [95% CI 1.60–1.72] and females 0.51% [95% CI 0.47–0.55]. New diagnoses significantly increased in older (5–12 years) males and females but not younger (0–4 years) children, from 2010/2011 to 2015/2016. The M:F ratio decreased in older children (4.1–3.0), but not significantly in younger children (4.2–3.5). Identification of older males and females is contributing to the increased in ASD in Australia and proportionally more older females are being diagnosed.