Acmena smithii *Lily Pilly* would be your best bet there, I have 12 along the cyclone fence to block out the neighbors.

They have been in for 10 months and the soil is the terrible as 20 years of the former neighbour dumping his rubbish over the fence on a vacant block has taken its toll on the soil, but with the rain we are having now they are starting to fly away..

I tip prune them every couple of months to make them more bushy.

Get rid of those cadargi (Corymbia/Eucalyptus torreliana) as quickly as you can! They're a horrible environmental weed and will only get bigger while you wait.
It would be a real help if you could tell us where in SE Qld you live. How heavy is your clay soil? Are you talking blacksoil here? Or something better drained like Toowoomba redsoil (which is also a clay soil)
Lophostemon confertus (Brush Box) would be a good, fast-growing replacement for the cadargi. It is bushy when young, then you could coppice it after ten years or so (cut it off 20cm from the ground) to force it to start again with multiple shoots, so it becomes a multi-trunked tree. Alternatively, leave it to become the upper part of your screen, because you have interplanted with something smaller. Resists light frosts and a fair bit of drought.
Elaeocarpus obovatus (Blueberry Quandong)is frost hardy and moderately drought tolerant. It grows fast if you can give it a bit more water while it's young. Water crystals under your plant when you put it in the ground, and a good layer of mulch, and the shelter of a plastic tree guard would all help. It can get to be a big tree, but in an open site like the one you describe it is a smaller tree with a bushy shady crown.
Trish

Why don't you try Elaeocarpus reticulatus (Blueberry Ash).
They should be good for 4-5m in your area.
If you need to lop them at 3m do it.
I have 6 of them screening a fence and they're being lopped at 2.5m.

Have fun
Roy