As you are in Australia, would this be the local Harpullia pendula, rather than Liriodendron tulipifera that most NH posters here would be familiar with? Both are known as "Tulipwood".

But a tree is a tree, so I suppose the treatment is the same...was the tree planted before or after the erection of the retaining wall?

any chance at a pic or 3 ...

i might understand removing a decaying old wall... but why not just leave the soil??

most trees roots are in the first foot or so of soil ... and i wonder how you are going to remove a lot of soil around a tree [width i mean].. but then.. you dont really tell us how much soil you are thinking of removing ...

and whats your definition of a semi mature tree ...

every leaf is a energy making machine.. so im not sure i would go removing a lot of them.. to facilitate all this ...

we need pix to really help you

ken

For the tree and my laziness, I'd probably go slow. First remove the bricks, then let the soil was slowly down to yard level.

More so if this is a 10 meter circle around the tree which is likely to contain a good amount of its roots.

Its winter down there, right? I am becoming quite the Australia-phile thanks to BBC news but don't know your microclimates at all. Is the tree dormant now?