We have spent over 15 billion on recovery recently, and that's just govt money; and just a few lousy million at the same time, on mitigation schemes.
Saying something can't be done when we've never ever tried, shows sheer lack of applied intellectual acumen!
Statistically, for every one dollar spent on mitigation, four are saved on recovery outlays.
And as one poster pointed out, we've simply replaced what was lost, when we should have spent additional money on raising road and rail links, and replacing bridges that wash away, with tunnels that don't; and, putting levy banks around towns that then turn them into Islands, whenever floods come again, etc/etc.
The Clarence flooded recently, and the levy banks built around Grafton, some years ago. Against the wishes of a small but very vocal minority, who like Chas, said it can't be done; have since paid for their seemingly modest outlay, many times over!
The floods destroy quite massive areas of the marine and estuarine environment.
And outlays for modest upland mitigation work, would provide nothing but benefit, and nothing but win/win outcomes in both wet and dry times.
No, we can't flood or drought proof Australia, but we can and should mitigate against some of the very worst outcomes!
If only to actually reduce the massive and endlessly repeated recovery outlays.
Rhrosty

Rhrosty:

...I’d take you to point on the view that upstream mitigation is the weapon of flood control with small gully dams as you propose. Using the Clarence as an example,(as you did), one of its main tributaries is the Mann River. I've seen flood debris on its banks at least eighty feet above normal river levels.

...There is little chance of mitigating these flows! The Clarence has three main catchment areas as well. If it floods simultaneously in all three catchments there would be little of value left in the Clarence valley to restore!

...The first priority in assisting flood victims is to maintain faith in an insurance system which evens out the loss across the whole community, and not inventing fanciful schemes which will never happen.

...The Insurance industry is in full scale retreat from its responsibilities in this regard, and is running free to that end!