Background: Low physical activity has been associated with increased fatness and deceased fitness. This observational study aimed to describe the magnitude, composition, and time-distribution of moderate-to-vigorous physical activity (MVPA) in Australian children. Methods: A total of 1132 10 to 13 year old schoolchildren completed a 24-h activity recall diary on 2 to 4 occasions. MVPA was defined as any activity requiring ≥3METs, including sport, play, active transport, chores, and other activities. Results: MVPA was higher in boys than girls (173 vs 140 min/day; P < .0001), higher on nonschool days than school days (166 vs 143 min/day; P < .0001), and decreased with age (9 min/day per year of age). MVPA consisted of structured sport (37%), active transport (26%), unstructured play (24%), and chores/miscellaneous activities (13%). Every hour of MVPA was associated with a reduction in screen time (26.5 min), non-screen-based sedentary pastimes (8 min), and sleep (5.5 min). The least active quartile of children were more likely to be girls (OR = 3.4), have higher screen time, and sleep more. From 4:00-6:30 PM on school days there were large differences in participation between high-active and low-active children. Conclusion: Findings suggest MVPA interventions should target girls, screen time and focus on the after-school period.