One of the reasons that the floods last so long in these areas is that the area is so low-lying, and is drained by rivers that flow very slowly on world standards.
So, for example, have a look at a map of the Murray-Darling river system, here:
http://en.wikipedia.org/wiki/File:Mu...-map_MJC02.png
The land at the headwaters of the Darling river, in the Darling downs in southern Queensland, is only 100m higher than the mouth of the Murray river near Adelaide in South Australia.
Many of Australia's inland river systems are similar, with water only flowing out of the rivers very slowly and flooding lasting a long time when it does happen.
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