Investor 13 – Cosmo‘s generally correct, but this is again one of those situations where the answer to your query is not straight forward.
The “rule” that’s generally applied in circumstances where there is no Special By-Law in place, is that the Owner of the courtyard is responsible for keeping the height of their tree/s within the stratum of their lot, but where that’s not been done (as in your case) then the costs for maintenance / trimming is apportioned between the lot Owner and the Owners Corporation.
The “common sense” apportionment formula that I use is relative to the stratum – where if that’s 3m and the offending tree is 6m, then the O/C commissions the work and the lot Owner pays 50% of its costs.
Don’t forget to have your O/C check with Council about tree preservation orders and any restrictions on trimming / lopping that may apply.
PS – the “stratum” of the lots in your Scheme will be shown in the notes on Strata Title Plan, and will be a specified distance (M) above and below a defined/fixed/ point on the property such the corner of the building’s foundation slab.