For those for whom a next to nothing levy is too much of a sacrifice let me explain.
The funds should not simply be used to build/rebuild homes willy nilly, they should however be used to provide assistance by way of living expenses. I would like to see relocation assistance as rebuilding in presently flooded areas is an obvious futility. State Govt should provide land that is clearly not in risk of flooding. Such land should only have to be paid for by the next purchaser of the home, the relocated families should only have to pay normal Council rates.
I'd imagine that most of those who built in the flood areas did so because local authorities failed to prevent building there. Local Councils need to be made accountable after this disaster & any future subdivisions must have a Council insurance & in areas of potential once in a lifetime flooding, high set homes should be mandatory.
This is merely a rough idea of what I think could/should be thought about & those with far more experience than I should have no great difficulty in making changes for future subdivisions. Unless planning moves away from status quo I'm afraid these places are simply waiting for the next disaster.

Why not have a State Govt Lottery so that the selfish can also contribute in building new flood-free homes ? I'm talking normal 3-4 bedroom homes that people can afford to keep instead of those multi million Dollar mansions that most winners have to sell for a fraction of their worth ?
We need ideas to make our economy snowball. There's enough money out there it just needs to be spent here.

It is probably a bit too late to think of abandoning the homes in flood areas. Most have been there for over 50 years, & up to 100.

A quick back of envelope bit of math tells us that even in their flood depreciated state the 15,000 homes effected are worth at least 10 trillion dollars. A hell of a lot of money to find to replace them.

It is probably a bit too late to think of abandoning the homes in flood areas.
Hasbeen,
Yes, a tough ask but at the same time the only other option is to keep getting flooded. I imagine the insurance companies will soon put a stop to covering these areas so, those who decide to stay also decide to be on their own.
I live in an area where I can't insure my boat because insurance companies decided that they can't cover boats in cyclone areas despite most boats being stored on land during the cyclone season.
As harsh & tough as it is I think necessity will stipulate that future subdivisions will be well away from today's high water marks. Should we expect more such severe rains ? I think it may be prudent to think so & even if the water rises not as high again it'd still be worth the outlays of widening watercourses on an on-going basis. This would definitely alleviate a lot of potential damage & loss.

individual,
There are also the options of moving houses, raising them and levies.
The town of Adaminiby in NSW was moved to higher ground above the water level for the dam, small scale but it can be done.
Inverell, NSW, suffered a bad flood some years ago and many houses were later raised above flood level.
Tamworth, NSW, is an example of successful levee construction.

I do wonder at the sense of building underground railways beneath flood plains.

individual: "We need ideas to make our economy snowball."

Here's an idea: the free market! This has been shown time and time again to be the best idea to make the economy bloom. And under the free market there are no government bailouts for people for do stupid things with their money.
Eg, if you build or buy a house in a *KNOWN* flood prone area then, unless your so wealthy that it doesn't matter to you, you're an idiot. And you are being rightfully punished by the free market when you lose your investment- the smart person who sold you your house is doing society a favour by sending you broke!

Under the free market, it's not the government's job to protect people from their own stupidity but rather to protect people from other's stupidity. (This also applies to violence/harm- it's the government job is to protect citizens from other's violence not self inflicted harm/violence.)