Lack of volcanic or seismic activity leading to mountain building over much of the continent for many millions of years means that what mountain ranges there were have been worn down by erosion to mere stubs of their former glory.The soil over much of Australia, especially the dry central parts, have not been renewed by volcanic activity for a very long time and are impoverished compared to soils from most other parts of the world.There are not many areas above about 1700 m, and even at Mt Kosciusko, the highest point on the continent, it is less then 3000 m above sea level.
In many places these seas were eventually cut off from the ocean and evaporation formed huge deposits which were later buried.
So added to this impoverished state of the soils there are vast areas where there are salt deposits beneath the surface which means when land is cleared for agriculture the removal of deep-rooted trees and shrubs mean the water table rises.
Endorheic streams that flow to basins of internal drainage serve more than half of the continental area.
The Lake Eyre catchment accounts for 1.3 million km The Australian continent is unique in a number of ways.
By the time the water table gets close to the surface it has passed through these salt layers and so causes salinisation in the root zone and eventually at the surface, which makes the land useless for agriculture.