The earliest evidence we have for human populations in Greater Australia is 60,000 years ago. However, some Indigenous creation histories state that they are totally native to Australia, and the earliest possible evidence of migration is likely drowned with the continuing rise in sea levels.

Archaeologists have noted two major changes in Australian flora and fauna coinciding with the migration of humans - the spread of oily fire-loving trees like eucalypts and the gradual extinction of the megafauna. However, the extent to which humanity played a role in these changes is greatly debated.

For instance, Indigenous Australians are still today noted for their use of fire for clearing land for a huge variety of uses, including the creation of ideal hunting enviroments, hunting, creating tree-based traps and many more. Most megafauna would have had no fear of humans, having not evolved to see them as a threat - it could have been relatively easy to hunt them.

On the other hand, the climate changed radically over 60,000 years, and climate change is known to cause mass extinctions in both plants and animals. Aboriginal Australians did not have the power to alter the weather.

Other changes include the migration of plants and animals for economic uses throughout Australia - fairly extensive trade routes, festivals and diplomatic gift-giving ceremonies ensured that useful items like seedlings or young animals could be spread. This is likely how the dingo reached every area of mainland Australia - it would have originally been introduced by Austronesian seafarers 1000 years ago, in the north.

When Europeans began to invade and explore Australia, they often commented that the landscape looked like the park estates of English gentleman - manicured lawns and artfully placed tree groves, etc. This was the result of millenia of Indigenous burning and planting, creating landscapes that maximised economic output and conformed to aesthetic values and cultural needs.

Non-Indigenous use of land has been incredibly destructive. Massive clearing of land, over-usage of water and the introduction of numerous foreign species of flora and fauna on one of the most fragile and unique landmasses on Earth have caused an enormous flood of conservation issues, leading to mass extinctions and environmental catastrophe.

The book you want to read for this is 'The Biggest Estate on Earth' by Bill Gammage. He goes into exhaustive detail over Indigenous land use and the colonial impact on land. Another good resource is Scott Cane's 'The First Footprints', which in one of its chapters discusses the arguments around the extinction of the megafauna.