I've had my eyes on this one for a while, but have been sick for several days, so sorry for the late answer.

To have an empire, you need a system of hierarchy and a sense of transferable land ownership.

Aboriginal Australians did not have a system of hierarchy. Although the role of women in traditional society was very less than equal, everyone was expected to be able to speak, and the words of the older and more experienced 'great men/women' (nowadays called 'elders') carried a lot of weight. There were no kings, no chiefs, no nobles - only people who distinguished themselves through their words or actions.

Aboriginal Australians did have a sense of land ownership, but it was not exclusive or transferable. Inheritance followed either matrilineal or patrilineal lines and was shared with those outside the family/clan group so long as they had the blood connection.

This meant that there were often wide-ranging social connections between different tribal groups across vast areas, which helped a great deal with trade and diplomacy. Totem and skin groups (moieties) ensured that marriages weren't local, meaning a wider connection beyond cultural borders was built into their social system.

Land was not transferable for two reasons, the first being that private ownership of property was not recognised in Indigenous culture, and the second being that there was/is a deeply spiritual connection between a person and their land. The land would become sick if the person became sick, and vice versa, and a person was expected to be born and die in their ancestral land. To die outside of it was to have your spirit lost and wandering, forever cursing yourself and your land. Even today Aboriginal Australians regularly express their belief that being away from their land makes both sick, and that 'returning to country' is healing in itself.

These two factors meant that it was inconceivable to steal from somebody else's land - especially since most Aboriginal Australians lived in abundance, their population density far below what their land could support.

War with other tribes was still an every day fact-of-life, but it was at a much smaller scale and carried a sense of crime and punishment to it, or cycles of revenge. Women were abducted (as they were the primary workers of the Australian economy), formal-but-limited duel-like battles were fought over honour, and tribal justice was meted out to trespassers, outcasts, adulterers and other villains. Large warbands and battles were rare, but did happen occasionally.

In fact, the conquest of another country was so unthinkable that many Aboriginal Australians believed that the invading British were actually their ancestors, returned from the dead, and thus entitled to live on country. Although not welcomed, the British did receive significant aid from the native Australians they sought to replace - a history conveniently forgotten in the 'white pioneer' national mythology. It was the concept of private ownership, of land and property, that saw conflict escalate, as the invaders shot the 'thieving' natives, and the natives punished the 'greedy' whites who refused to share, leading to the starvation and homelessness of their people.

Much of this can be read in Henry Reynolds' The Other Side of the Frontier. If you'd like a heavily condensed and military-focused version, try John Connor's The Australian Frontier Wars.