If you think there is an issue with the wall, then the next step would be mediation and then NCAT. Under section 232(2)
To support your case at NCAT you would need to have a report done on both the tree and the wall. Hearsay and opinion do not sway tribunals.
If you win you may be able to recover these costs as the reports could easily have been commissioned by the OC, to either infirm or rebut your opinion.