Answer:
They killed lots of indigenous Australians and claimed the land as their own saying it was uninhabited when they arrived. Always has, always will be aboriginal land.
European occupation resulted in the introduction of many new animal and plant species, which ultimately led to the extinction or near-extinction of many, many native species. The balance of the land and the environment was lost, particularly once tree-clearing started and agriculture was introduced.
Agriculture in inappropriate places (such as above the Goyder Line in South Australia) has led to increased desertification. Redirecting natural water channels means that many areas are more prone to flooding, while redirecting water away from main rivers such as the Murray has led to rising salinity levels in the river.