It depends on the kind of crops you have. The most used crops in the region (south east Asia and Oceania) were not suitable for agriculture in the temperate south east of Australia. At least that is my (admitted very limited) understanding of the situation.
I live there man, and sometimes it can be as dry as the Sahara and as cold as Scandinavia. In some places (Tasmania, the very thin nice climate coast, etc) you could support some hardy crops but they'd have to be able to stand the cold/heat and the somewhat common thing called a drought. The aboriginals were too "skilled" in their hunter gatherer lifestyle to abandon it anyway.