Water safety knowledge levels of Australian children are poorly understood. Royal Life Saving developed an online National Water Safety Quiz (NWSQ) as an interactive means of determining water safety knowledge amongst Australian primary school children (ages 5 to 12 years). Over a period of 8 months, a total of 4,215 children participated in the NWSQ. The NWSQ identified areas of water safety where knowledge was poor including the topics of cardio pulmonary resuscitation (CPR), swimming, and river safety. Children achieved a better result as they aged. Females out-performed males overall and specifically from ages 10-12 years. Children from independent schools performed better. This research is the first of its kind to measure water safety knowledge at a population level for primary school children, using an online web-based tool. Understanding children's water safety knowledge is important as it enables the tailoring, delivery and evaluation of programs which help to reduce the burden of drowning.