Why didn't the native inhabitants of Australia develop an empire?

Throughout most of the world, ancient civilisations developed into empires. All of Europe, Asia, South America, and much of Africa gave rise to flourishing civilisations. I must admit ignorance as to whether or not North America had any empires apart from the small region occupied by the Mayans. However Australia did not have any empires until Western powers colonised. The native population lived here for around 50,000 years, and there is no evidence that they ever built structures of any permanence or developed technology beyond that of the hunter/gatherer society. Why would this be so? Why didn’t they build a civilisation based around cities, as cultures in practically every other part of the world did?