I have a sloping rock garden with about 40cm of soil covering a rock
shelf... its really the only place in the garden I can put a tree for some
summer shade so I'd like to put a 8-10 metre squiggly gum in there (to match
an existing one) if at all possible.

I'm worried however that the root ball/roots won't have enough room. The
soil is right over the rock shelf, so the roots can wander as far as they
like and within about 3 metres from where I'd like to plant it the soil will
be a good 2 metres deep at least, but basically my question is whether or
not the first big storm will knock the tree over once it gets larger.

(I can't plant it directly where the soil is deeper as it will be in the
way... and the other squiggly gum is in deeper soil on the side of the rock
shelf)

Any ideas?

Thanks,
Dave.