If you are considering entering one of the Special Categories refer to the sites that they list on their own websites. In many cases they can direct you to excellent resources and provide some stunning starting points.
If you are looking at entering in the general Year Level categories then you need to consider what you believe a ‘Defining Moment’ is. Once you have selected that moment then carry out research around the topic, looking for interesting or quirky events or people that helped establish that period, day or event as a ‘Defining Moment’ in Australia’s history.
Use books, interviews, old documents, photographs and other similar sources rather than just information from the internet. You may find that your family has an old collection that might head you in the direction that you want your research to go.
Talking to people about the topic might also give you some ideas. Depending on their interests they may come up with historical, political, social or sporting events that have stood out in their mind. As a fairly young man I still remember being allowed out of class to watch the first moon walk; if you looked at that event, what was its effect on Australia as a nation? Did it have any effect beyond the United States of America. Other events such as Olympic Games, Commonwealth Games, a decision to become involved or withdraw from an external military conflict, to offer an apology to the indigenous, or to the thousands of young forced British migrants could all offer you a ‘defining moment’ from which you could begin your research.
Remember, try to be original and look at areas that might not necessarily have been studied in depth before. Your own community or its records could offer you such a point.