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There are two ways to view this subject. The first argument is to use our engineering capabilities to invest in flood mitigation infrastructure, for example Wivenhoe dam. However, if Wivenhoe was designed to operate at a normal capacity of empty (which obviously gives the maximum benefit to flood mitigation) then it would provide no benefit with regards to supplying drinking water or supplying electricity through hydro electric means, which both offer economic benefits. The result, a compromise where the dam is built with 50% being used for drinking water and hydro electic supply, the next 50% up to the spillway being used for flood mitigation, a further 25% capacity while going over the spillway to cater for a 1 in 100,000 year event.
However, even if you were to invest the maximum in flood mitigation infrastructure, there are some areas where dams are simply not practical, and the brisbane river through brisbane has several sources, if you look at a map wivenhoe might affect a quarter (?) of the catchment area. This is where you have a look at the other argument - accept that floods will occur, and learn to live with it. Relocate homes and businesses that are in low lying areas, and restrict buildings in the peripheral areas to buildings that are flood resilient like the classical queenslander house on stilts. Somehow I don't see the pollies as having the balls to relocate tens of thousands of people though.
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