Tunneling the flood waters under the city wouldn't work unless you had gigantic pumps at the sea-end because the River is still tidal (ie: close to sea level) at the city. But of course we simply don't have pumps big enough for this.

thinkabit,
Not tunnelling under the city, there’s enough tunnels there now, but digging/tunnelling a bypass waterway.
Tunnels would be needed unless resumption of flood free homes could be tolerated, such a diversion needs to be straight for maximum effectiveness.

Can you not see the hypocrisy in this?
thinkabit,
No, I only see your insipid objecting instead of thinking how to reduce the degree of future flooding & damage costs.
Do you really expect me to put forward a fully comprehensive plan on a solution between each post here ?
I'm simply putting forward ideas that could possibly be feasible.

Issy, I am shocked at your suggestion. "TUNNELLING".

Where exactly, MY HOUSE!, which was safe and dry at flood time. Is my house in your firing line?

MY SUGGESTION!

BUCKETS.... available from Bunnings at 98c each. Combined with the frugal use of bathtubs the flood problem should be no more.
Should the bucket brigade fail, then Plan B can kick in! MOSES, I'll hold the umbrella whilst he works his miracle.

Plan C is to ridiculous to mention.

>>"Do you really expect me to put forward a fully comprehensive plan on a solution between each post here ?"

No, but it would be nice if you gave more than a fart's worth of consideration to the cost and practicality of your ideas before you post them. A child could tell you that they don't make economic sense, every idea of yours so far would cost more than the damage it spares. Nor are some practical/possible from an engineering point of view.

>>I'm simply putting forward ideas that could possibly be feasible.
No! There not feasible!

As I said in the last post, the cost of flood damage per flood is really quite minor in the big picture. We're talking low billions for these current floods in QLD and NSW- which for a state or country is not much. To repeat, we spend about the same amount *every year* on coffee's in Australia as what these recent floods are projected to cost. And floods of this magnitude occur only a few times a lifetime. For a solution to be feasible it has to cost less than the flood damage it prevents.

We do have a common man-made solution to floods- they're called DAMS. The reason dams work is because you don't have to build much to get a large return on the amount of water they prevent/delay from entering the flood zone. The dam itself can cover 10's to 100's of square kilometers in surface area for only 100's of meters to a few kilometers of dam wall.
For example, Lake Wivenhoe has a surface area of of about 11 square kilometers for a dam wall that is 59m high (at its highest) and 2.3km long. It has a flood mitigation capacity of 1.45 million ML = 1.45 cubic kilometers.

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However, the problem with flood mitigation dams though is that you need somewhere to build one. This means:
1) you need the right topography - the valley shape of the river has to be such that the dam will hold back a lot of water
2) the dammed area needs the right geology, especially where the dam wall is. It has to have the right sort of rock because water and earth interaction can cause massive geological instability problems.
For example, enough water in the wrong place can even trigger events like fault line slips (earth quakes). This, I expect, wouldn't be such a problem in Australia, but relevant to places with active fault lines such as New Zealand.
3) you need a large area of land that's cheap to buy without many buildings/man-made infrastructure up stream from the dam wall. Its pointless building a flood mitigation dam if you have to forcefully resume more buildings than what get flooded
4) You preferably want to be able to build the wall at a pinch point on the valley to minimize the length of the dam wall to reduce its cost

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Paul1450: I love your bucket idea, it's so simple and obviously perfectly feasible. However, I would recommend that you don't go down the 98c wire-handled bucket route, but upgrade to the $2.50 buckets with the black plastic handles. These buckets are tougher/more durable- I'm speaking from experience here.