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.