Hi lizgleeson. I had hoped someone with high expertise in this sort of thing might have responded by now. In that person's absence, here are my thoughts but keep in mind I'm a long way from being an expert in Eucalyptus dieback.

The natural range of Angophora costata is a long way from south western Queensland where you live so your trees are being subjected to quite different conditions from what they would normally experience. It's quite possible that your trees aren't coping with these unnatural conditions (different soil, temperatures, climate, rainfall) & this is stressing them no end. In fact, the photos you provided remind me of Tuart Gums Eucalyptus gomphocephala planted in SA where I live. After a few years they become quite stressed & open to borer attack to which they readily succumb. Have you noticed any grubs in holes in the trunk or branches?

From my limited reading on Eucalyptus dieback fungi seem to be a major cause of cankers in Eucalypts. Have your trees been subjected to consistent humidity? (Unlikely in south western Queensland, I guess). Have you been spray irrigating them? (Again, unlikely in s.w. Queensland but might depend on water availability).

Another cause of dieback is reported to be defoliation of the canopy, presumably by bugs or larvae. How this works I don't know but the canopy in the last photo looks severely ragged (no offence!). Have you noticed small leaf-eating critters in the trees' canopies? If so, the most effective remedy is to encourage their predators (often native birds) by planting &/or encouraging the regeneration of local tree & shrub species near your Angophoras. However, this will take some time & it may be too late for your trees.

Your trees could be severely cut back & may, repeat, may become reinvigorated & shoot afresh. You could try chopping them off near the base to see if they send up new shoots.

My preference is to plant trees which are local to where you live. They've survived for many thousands of years where you live & are adapted to your local soil & climate so if your Angophora costata don't survive you could replace them with local species. You'll find that local species are more likely to attract local birdlife so this could be a bonus for you.

If my thoughts are way off beam then let me know but I must admit my curiosity about your problem is high. Hopefully, we'll hear from an expert soon.

Hmmm I had considered a fungus but i'm starting to think it may be insects. Distribution of Angophora Costata extends inland to St George where we are so they do grow out here. And the majority of our trees are incredibly healthy and huge! It's only affected 3 out of 20 trees and the one pictured is the only one to be severely impacted. Typically it's very dry here so humidity isn't an issue and aside from rainfall, we only water them via dripper line when we haven't had rain for a while. I think I'll try cutting them right back and see what happens. My only concern is the possibility of it spreading, whatever it is!

St George seems to be within the range of Angophora costata attested to by the fact that the majority of your trees are bright & cheerful. The fact that only three are badly affected might suggest that their roots are affected by a specifically located pathogen or a toxin in the soil. Are these three trees next to each other or randomly dispersed among the others?

I'm interested to learn what you discover from observing the leaves, trunks & branches for insects & borers.