Hello concerned,
You mention in your first post that it was ‘noted’ that the tree was the owners responsibility. It doesn’t sound like there a bylaw enacted to enforce this?
While you say the council has a record that the tree is the owners responsibility, that is probably more to do with the fact that the tree is registered with council (being 25m+) and has to be in the ground ‘somewhere’ on enclosed land rather than an OC bylaw transferring the responsibility to the lot owner: councils – particularly the department that looks after trees and tree preservation – generally are not concerned with that.
If there is no specific bylaw or absolutely specific clause in the sale document then really the absolute bulk of this tree is always going to be common property. The owner is responsible for everything from the ground up to about 3m; the remaining 22m with all the branches and limbs + the roots that extend everywhere else are the OC responsibility.