Actually, there might be a solution by way of climate lunacy - zero emissions and so on - which will impact on country areas much more than it will the cities by way of lost jobs, fewer farting farm animals (there is no end to what the nutters want to see disappear). There will be no one left to be affected by flooding.

Paul,
The solution in Toowoomba is as obvious as any where else, if it floods then it’s in the wrong place, shift it or lift it.
Tamworth, NSW, is a prime example, the city is mainly built on the river’s flood plain, as the city has expanded the newer developments are flood free as they are on what was considered undesirable land.
The very costly levies now protect the low areas but if they are breached . . .
Legend has it that the local tribe told the original settlers that they were building in the wrong place but more intelligent minds ptevailed.

Indy, remember our Seniors National Service motto, "Have shovel will travel".
Paul1405,
That's your motto, mine is "Have brains to think with, don't need shovel !"

How big is the

Elon Musk’s Not-A-Boring Competition winner announced
http://roadsonline.com.au/elon-musks-not-a-boring-competition-winner-announced/

>>A team of students from the Technical University Of Munich has taken first place in “Not-a-Boring Competition” – a tunnel construction competition designed by Elon Musk’s The Boring Company to help improve the speed of tunnel boring machines (TBMs).

The team, named TUM Boring, was one of the only two teams out of 12 competing to pass the strict safety inspections to qualify for drilling in the 30-meter competition segment. Swissloop Tunneling from ETH Zurich was the second team to pass the safety inspection and came second in the competition.

More than 400 teams had applied to take part in the competition, with 12 finalists invited to Las Vegas to put their TBM inventions to the test. After the strict safety briefing, however, only two of the teams were admitted to the actual tunnel boring round.

Competitors were required to drill a tunnel measuring 30 metres in length and 50 centimetres in diameter in around three hours with the TBMs they had built themselves. TUM Boring’s TBM drilled a 22-meter-long tunnel, while Swissloop Tunneling managed to dig a tunnel more than 18 meters long.<<

It may not be impossible, but it would be a big undertaking.
150km would take 2.3 years, even for a 50cm hole
But if you were able to have multiple machines doing multiple sections..
Or even one coming in from each each side for example, with junctions and access tunnels maybe.
It seems the technology here is getting better, and we're moving from non-viable to possible.

Don't they currently direct some of the excess to Atkinson Dam from Wivenhoe. Wivenhoe Dam seems to be located amongst hills 50 kms north west of Brisbane- surely there's a way of redirecting some of the flows elsewhere. I'll have to look at an elevation map some time of south east Queensland. Maybe they can direct some of the water in the direction of Emu Creek of course it depends on the elevation but I'm not sure 100 miles of tunnel is required- perhaps a few strategically placed explosives. Just looking at the map. There seems to be a few dams close by

If excess water can't be diverted then it must be accommodated ! Widen/deepen riverbeds, widen/deepen catchment tributaries, bore tunnels where viable/practical/sensible etc.
Stop looking at such projects as from the start money-making infrastructure. Look at it as infrastructure which will snowball environmentally & economically once it has started to move.
It has to be a Govt stimulus project to start off with which will very quickly create the base for economic sustainability as people move into areas alongside the projects' corridors.
Flood proof Real estate being the main drive. Diverting water will once again filter the outflows into the oceans & that too will have all sorts of benefits. Flood land needs to be created to prevent flooding but eventually this water will flow to where it has to flow into the ocean. More canals or wider riverbeds and/or diversions are the only mitigation available and, they create employment without the damage bills !